That suggests a difficulty in the Sonos devices staying on the network. It could be some sort of wifi interference , or less likely, a duplicate IP address issue. I’d certainly read the linked FAQ for potential solutions, as well as doing a network refresh, by powering off all Sonos devices, rebooting the router, then waiting two minutes before plugging in the Sonos devices again. 
 

If neither of those suggestions seem to help, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. They should be able to ‘see’ in the diagnostic what is causing the speakers to drop off your network. 

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.

Thanks for the response. 

I think it's actually my mobile phone (a OnePlus). Why? Because on my ipad or my wifes Samsung the Sonos units are visible within the Spotify app. It's only the app on my phone and deleting and reinstall it does not fix the issue. Very frustrating. 

Although no idea what is causing the issue 

All the more reason to submit a diagnostic, and call in, so that the Sonos folks responsible for changing it have a solid case to look at when doing so.