Your profile states you own Play:5 (Gen 1)'s. These can only work on Sonos S1 software and with de Sonos S1 app. They cannot be commanded by the S2 app and cannot be grouped with devices running S2 software: https://support.sonos.com/s/article/4786?language=en_US and https://support.sonos.com/s/sonos-s2-overview?language=en_US

full history:

I owned 2 Gen 1's, one broke and I was offered a new Gen2 to replace it.

+Since than I bought a second Gen2.

=Total of 3 series 5 speakers.

Wanted to go to S2, but needed to replace the Gen1. Took up the offer to get two brand new series 5 Gen2 by upgrading my existing two series5 gen2. I was expecting the existing Gen2 series 5 to continue to work, ending up with 4 series 5 gen2 speakers. I have yesterday ‘decommissioned’ the one remaining Gen1, it is unplugged and no longer part of the system.

When I now login tot sonos site, it only shows an old Gen1 (now no longer in use) and the two brand new series 5. The two upgraded series 5 (gen2) no longer show up in my inventory….

Hi. I think it would help if we used standard terminology. There is no Series 5. The speakers are called:

Play:5 gen 1

Play:5 gen 2

Sonos Five 

Could you have another go at explaining what you have please?

Apologies:

• 2x brand new Fives: bought using the upgrade offer
• 2x Play:5s (gen 2): opted to úpgrade these, expecting to continue to use them
• 1x Play:5 (gen 1): no longer used, unplugged
• upgraded the system to S2 yesterday
• (also own 2x play1, that both work with S2 and show in my rooms list: no issues)
• Sonos S2 app: rooms show two brand new Fives, and 2x play1, but not the two Play:5s gen2 that I already owned.

Moderator Edit: Edited for technical correctness, just to get things moving in the right direction.

• but not the two Fives gen2 that I already owned.

Those must be the Play 5 (Gen 2)'s, that are indeed both S1 and S2 capable. Are you sure they have updated to S2? Are they visisble in the S1 app if you would install that?

If the Play 5's are indeed still visible in the S1 app, I would need to check. (not at home right now). I will verify tonight. 

If indeed visible in S1 (which was the status two days ago), adding the Play 5's to the S2 should also update them to S2 right? Or should I do that from within the S1 app?
 

The steps to update S1 speakers to S2 are mentioned here: https://support.sonos.com/s/sonos-s2-overview?language=en_US

If the Play:5/gen2 are visible in the S1 app then the speakers are connected to the wrong/old system.

1. Uninstall the S1 app. You won’t need it any more, and it could confuse things. 
2. Factory reset the Play:5s.
3. Go into the S2 app and add them. They will update themselves to the correct firmware automatically.

I suspect the problem has arisen from one or both of the following:

1. The way in which you transitioned from S1 to S2 - @106rallye ‘s conjecture being a very plausible example of this
2. Not factory resetting the Play:5 gen 2’s correctly

I am surprised the Play:5 gen 2s have disappeared from your Sonos account, as this would normally only happen if they were added to a different Sonos account…. but let’s park that thought for the moment.  We need to know whether the Play:5 gen 2s are still in your S1 system.

Edit: You mention that you reset the Play:5 gen 2s.  You don’t actually say that you have re-added them to the system using ‘Add Product’, or at least tried to.  Did you do so?  If so what happened?

That appears to be where the OP is stuck as they originally stated:

Tried factory resets (after which they are visible to the app, but warning appears: 'they may not show up in your system, just unplug and plug back in', but never actually show up in my system. 
 

Maybe try connecting the Play:5 to your router by Ethernet whilst you attempt to add them to your S2 system.

OK, S1 uninstalled on my phone.

factory reset performed on both Play:5’s yesterday. got the correct orange/white, and than green lights on both speakers. Added them in S2, seemed to work, until the very end, when I got message “..has been added, but may not be visible in your list, unplug/replug". But they never became visible in the list, not even after unplug/replug to electricity. I even renamed the (new) Fives that were visible to ensure there were no name overlaps that might be causing the problem, to no avail.

Now that S1 is uninstalled, I will redo the factory reset and try to add them again tonight.

As for Sonos account: In my sonos account the two Play5 speakers that I used for the upgrade are no longer visible.

Sit rep:

uninstalled S1, factory reset both Play5s 

used S2 to add product

• it finds the Play:5 
• I select ‘Add’,
• "to continue let's connect your mobile device to your Play:5's temporary network: Sonos-Aw0BBwAC...” choose Continue
• following that the Play:5 connects to hy home network ‘Connecting to PLay:5 to <homenetworkname>”. This takes about 4-5 minutes...
• “Your Play:5 has been addedm but may not appeat...unplug...to finish setup from the system app”
• following that, the Play:5 is nowhere to be found.

I tried conecting the two Play:5 directly to my network with a cable. Now each one shows up in my system list, after I have used the ‘Add” functionality. When adding them, they show both as 'this Play:5 is not registered”. After registering, it is in the list and plays music. Once the network cable is removed, neither speaker shows up in the list anymore.

Can you think of anything which would prevent the speakers connecting to your WiFi? MAC address filtering perhaps?

everything was working fine, until I activated the two new speakers. After that, it appears as if the new speakers took over, and the old play:5 speakers were disabled. The new ones work fine (as do the old play 1 speakers, that were not ‘upgraded’. Can the upgrade have resulted in the old two speakers being disabled somehow in favor of the newly activated speakers?

No. And they work when wired. Something else is amiss.

@Fran D - Which router are you using and have you rebooted your router to see if that makes any difference? I presume Sonos is setup on the 2.4HGz band rather than 5GHz as you have Play:1s, but please confirm this is the case.

Also take a look at Supported Wi-Fi modes and security standards to see if anything there stands out as to why your Play:5s can’t connect wirelessly.

Many thanks to all for your suggestions. I will reboot the router tonight when I get home. There have been no changes in the wifi system: one moment the speakers worked, the next (after installing and adding two new speakers) is was no longer possible to see or add them to the Sonos system...

i can indeed confirm 2.4HGz band

If you wire just one of the 'original' P:5s, is the other original P:5 visible when still wireless? Give the system a few minutes to reconfigure before you check. 

Do you have a simple network with just a router, or a mesh, access points etc?

big house, big WIFI system with modern Ruckus access points.

I will connect one P:5 with a wire, and see if the other -wifi based- (also) becomes visible.

Will report back ...

Ah, Ruckus. Known to cause problems for Sonos. Search this board.

Here’s a thread to get you started:

Thank you Ratty. Most interesting: still does not fully explain why it was working before, and suddenly stopped (maybe Ruckus system overload ), but I will try this tonight: 

It is recommended by the installer and Ruckus to connect one of the speakers through the wire and connect the remaining ones wireless if desired.

It’s very unlikely to afflict both simultaneously, but Play:5 Gen2s are susceptible to wireless card failures.

When one of mine failed I was able to see that its ‘PA rail mvolts’ was ~0, when it should have been about 5,000 (i.e., 5V). This value is available via:

http://<speaker_ip_address>:1400/status/proc/ath_rincon/status

Great news!

Plugging one old PLay:5 speaker into my home network via wire, leaving it in for a while, made both Play:5 speakers visible to the Sonos system after a bit. After a little while, I removed the cable, and they remained visible though wifi in the Sonos system list.

However, I later relaised that I could not play on the old speakers, whilst playing on the new ones at the same time. I solved this by leaving one of the speakers connected by cable to mynetwork. Luckily no big deal as the house has several network sockets (but not overywhere, which is why I bought the wireless speakers in the first place). I think this is also what Ratty’s link to the Ruckus website suggested, leaving one speaker wired up, solved my problem. 

Very happy with all the excellent suggestions I received on this platform, great stuff!! Truly. 

Keeping one speaker wired means that your Sonos devices are communicating over SonosNet rather than your WiFi.  This is how Sonos was originally designed to operate.  For those unable to wire a speaker there is the Sonos Boost.

I’m pleased you have a solution.