It may be that your controller is having trouble making a constant connection to your speakers. Try unplugging all Sonos devices from power, then reboot your router. Wait a couple of minutes, then plug back in your Sonos devices. Again, wait a couple of minutes before testing. You can also tap to either side of the slider for smaller adjustments.
If you’re still having trouble, 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.