For how to group, click on the underlined word in my reply. I ‘think’, based on your comments in your original post, that you’re doing it right, but it doesn’t hurt to be sure.
More troubling is your inability to play anything at all on the PLAY:1s. Beyond @Stanley_4 ‘s excellent point of making sure they’re running the same software, I’d be worried as to why they’re showing up in your controller app, but you’re unable to play anything music on them. I think at this point, I’d recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.
There are a score of reasons that might be causing this, the solution mentioned is the easy one.