The Associated player is the player that responds first as the controller initializes. You have no control over this. The controller is not playing the music, it simply starts music playing through a player.

If your issue occurs when you Group the players, Group order can be important if network connectivity is on the edge. As you build the Group the first player becomes the Group “Coordinator”. All network traffic for the Group passes through the Coordinator. In your case I suspect that the PLAY:1 is struggling a bit with its connectivity. Building the Group starting with the wired PLAYBAR will reduce traffic through PLAY:1.

I suppose that, if the PLAY:1 is on the very edge, the minimal extra traffic created by the players reporting to the controller through PLAY:1 pushes over the edge if the controller is associated with PLAY:1 when it is the Group Coordinator.

Interference could be an issue and this could be intermittent. Keep a log of issues. I once had a big issue and after a little logging it was clear that only some evenings were troublesome and Sundays were nearly impossible. I slipped a copy of my log, along with a polite note, under my neighbor’s door. A few days later the annotated log returned. She sometimes traveled overnight for work and called mom on Sundays. She was using a cordless phone system known to cause severe WiFi interference. I was able to physically rearrange my system and avoid her phone. Issue resolved.

Set your router to use 20MHz  channel 1, 6, or 11, different from the SonosNet channel.

Thanks @buzz, that info,  certainly makes sense, my Sonosnet is on Channel 11, all 2.4ghz Wi-Fi is on Channel 1, but I’ll check the frequency.

I can hardwire the problematic PLAY1.

I haven’t noticed any of my other Sonos Players ever being the Associated Player , I have 12 sonos devices, I just wonder why this PLAY1 and PLAYBAR are picked by the system as the associated product ? As neither is in a Central part of my house.

Anyway thanks for the advice, I’ll keep a log 

Technically, you could use 40MHz channel 1 if SonosNet is on channel 11. 40MHz channels spatter over an adjacent 20MHz channel.

Be open to the possibility that there is intermittent interference on channel 11. If this is the case, moving SonosNet to channel 6 might help. However, since you can wire PLAY:1, I don’t think fussing with channel assignments is worth the effort.