AC05,

Try this perhaps…

1. Remove the Spotify Service in 'Settings/Services’.
2. Reset the Sonos App in 'Settings/App Preferences'.

App will close and ensure it is fully closed. Now open App and choose the option to ‘Connect to existing System’ 

Go back to ‘Settings/Services’ and add the Spotify Music Service again and login with the same credentials as those in use in the Native Spotify Music App installed on the controller device.

If that doesn’t resolve the issue, then also see below…

Quite possibly you have a weak WiFi signal, or wireless interference around your Sonos devices. Maybe try a different ‘fixed’ non-overlapping 2.4ghz WiFi channel on your router (either channel 1, 6 or 11), ...or better still, consider cabling a single Sonos device to your router (if practicable) to move all your Sonos products over to SonosNet.

If you do decide to opt for SonosNet, which I tend to recommend to family and friends etc; then also do these two additional things:

1. Set the SonosNet channel in “Settings/System/Network” of the Sonos App so that it is at least 5 channels away from your chosen router 2.4ghz channel.
2. Remove/reset the WiFi credentials in the Sonos App “SettingsSystem/Network/Wireless Setup”. These are not needed when running the devices on SonosNet.

One further useful thing I would choose to do, is to add the Sonos IP addresses to your networks' DHCP Reservation Table… this is an entirely optional, but worthwhile, step.

Hopefully that will then resolve your streaming music service dropout issues.