Is there an error message?

Yes. “Unable to connect. Try again later”. 

When does this error appear? As you open the App? After the App has been sleeping for a while? When you try to play in a Room? Only when you attempt to Group?

Only when attempting to group. 

Try changing the Group membership and the order that you build the test Groups. ie. 1-2, 1-3, 2-3, 2-1, 3-1, 1-2-3, 2-3-1, etc. The first player in the Group becomes the “Group Coordinator”. All network traffic for the Group flows through the Group Coordinator. Likewise the Left speaker of a pair is the pair’s Coordinator. Obviously, a soundbar will be the Coordinator for a surround Room. If the Coordinator is struggling for connectivity, play will suffer.

Turns out one of my play 1s needed a firmware update. Problem solved. Thanks for your help. 

I’ve seen similar. I think that a general query goes out: “who needs an update?”. If a unit is slow responding for some reason it will not make the list. It’s not well defined what will happen when the system later stumbles across this stale unit.