I’d suggest the next time it happens you quickly submit a diagnostic, wait until it clears and submit another. Then contact Sonos support with both diagnostic numbers (don’t post them here) and get them to look into your issue.

This sounds like a local network issue, with the system not always connecting to the controller ‘instantly’. While there is some minor delay, I’d suggest it isn’t supposed to be that much of a delay. 

Beyond the good advice given by @Stanley_4 , I’d also try a network refresh, by unplugging all your Sonos devices from power, then rebooting your router. Once the router has come back up, then plug back in your Sonos devices, wait a couple of minutes, then test.  At worst, it won’t change anything, but it may help. 

Looks like it’s resolved. It was network interference. Changed the 2.5Ghz and 5Ghz WiFi channels and the app connects immediately.