Once you’ve decided that the issue must be e...] or cannot be e...], you are likely to be blindsided.

How are you adding them?  If you are following the instructions for setting up a new system instead of setting up a new unit, your speakers will disappear.  You add to a current system by going to Settings > System, and choosing Add Product.

That is how I did it. Setting - System - add products . They were added and upon returning to use added and existing products products were gone.  - I did it correctly. Sonos is flawed. FIX THIS SONOS. It happens to SO many and you point to user error and it’s NOT!!! 

@HKM 

If you’re only seeing one product at a time, it’s likely your router is having trouble with multicast. Multicast is a type of network traffic that involves sending data from one device to multiple others at the same time.

While multicast is basic network functionality and should be supported by every router, it's not something that's leveraged in most home network environments, and it's pretty likely Sonos is the only thing you own that needs it. That's why it might seem like "the network" is fine, but something is still actually wrong.

If that’s the symptom you’re seeing, check your router’s advanced settings menu for a toggle that says “IGMP Snooping”. If that’s off and you turn it on, there’s a solid chance that the problem will go away like magic. 

It still could be something else going on, but if there is a “quick fix”, that would be it. Other problems could be wireless extenders getting in the way of communication, a second router in the wrong mode, or you’ve got too many Sonos products (a good problem to have, of course, but the maximum number of units on a single system is 32).

Hope this helps!

ITS NOT MY ROUTER! I have 8 products and 5 will show up then 6 but it might be a different one that came and now one is gone. It’s 100% a Sonia flaw that SO MANY PEOPLE COMPLAIN ABOUT! This isn’t a wifi thing and the fact that Sonos can’t take responsibility and fix their flaw adds to the frustration! I think this is the final time that I have learned my lesson and will no longer buy another Sonos speaker because of the amount of time and effort it takes as soon as you buy one and go to add the whole entire system crumbles. Sonos is just a house of cards that cannot play nice with one another. Pathetic 

Sean M you’re clueless and I would really expect more from a Sonos employee. I bet you have this problem every single time you get a new speaker as well but Sonos doesn’t want to address that it is their issue and that the network that they have created and how they register new speakers and electronics within their system cannot handle the addition of new speakers without disrupting the entire system. Shame on Sonos 

Sorry you’re having problems, @HKM 

My experience has been very different: a stable system, to which I’ve added over the years. In the time of my ownership I’ve added several speakers without issue, gone through numerous trouble-free updates, and very rarely experienced any issues. 

I’m not alone in this. I’m not posting this to upset you, but to say that despite your certainty that Sonos is at fault, my experience suggests otherwise. 

Good luck in resolving it - if that’s what you are still trying to achieve. If so, if I were you I’d power down all speakers and router, restart router and wait for it to fully restart (including wifi), then power up Sonos devices, one at a time. After that I’d set static ip addresses for the Sonos kit. If you look on these forums, that has fixed a significant number of problems that have been posted here. 
 

PS in anticipation: No, I’m not a Sonos employee; just a satisfied user. 

I’m not experiencing these issues. 

I’m experiencing these issues too. This should be a consumer friendly technology and it is far from it. When it works, it’s great.  But beyond frustrating to have these problems and spend so much time here trial and error 

@Sean M my router settings don’t have this option. Is there anything way IGMP Snooping could be described?

Here is an article about IGMP Snooping. I’m sure that it’s a bit more technical than you’d like. The bottom line is that SONOS needs to be able to “multicast”. Simple phone/pad/computer access to the Internet does not need to multicast. SONOS units need to be able to chatter with each other. This is a potential reason why people are noting that their network was “perfect” before SONOS arrived -- therefore it must be a SONOS issue.

It’s possible that a wireless router is configured for IGMP Snooping (or not) by default and user access to the option is omitted in an effort to make the user interface appear “simple”.

We may be able to offer more specific advice if you provide more detail about your network. Please include model numbers.