Probably some sort of wifi interference, but that’s just a guess, there’s really not enough information in your post to hone in on anything more likely. I’d recommend wiring one of the PLAY:5s temporarily to your router with an ethernet cable, wait a couple of minutes, then plug in the “new” one to power. It “should” be seen at that point, except if the software running on it is too old to be recognized. Which it could be, since we’re talking about a PLAY:5 which hasn’t been manufactured or sold in years and years. 

If it’s the software that’s blocking it, you’ll likely need to call in to Sonos to get it updated to match your current PLAY:5, so that they can “talk” to each other. 

Note that a Sonos Five and a PLAY:5 can not be paired….they have to be the same. 

Your profile says you have a Gen1 Play:5. It can only pair with another Gen1 version (if you mean add into the same room?). Or, you can group speakers designated as different rooms, but they need to be on the same S1 or S2 system. 
 

Are you using the S1 or S2 app on your phone? 

Hi @Carl Ryder 

If the Play:5 you’re trying to add has never been part of your system before, you’ll likely need to factory reset it before you’ll be able to add it to your system.

Reset your Sonos product

Adding a Sonos product to an existing system

I hope this helps.