Have you tried simply deleting the app, and downloading a fresh copy from the Google/Android store, making sure to ‘Connect to existing system’ when prompted?

If so, or that hasn’t fixed it, I would 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 phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system