It’s really hard to tell with such little information, but I would suggest 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 Sonos system and network.
I assume that you are playing a turntable or similar through CONNECT’s Line-In. Be sure to check for silly things, such as an RCA plug not firmly seated in its jack.
A not so silly issue is network connectivity that is on the edge. SOTIFY requires less bandwidth than playing a local analog source. It’s possible that SPOTIFY can sneak through, but Line-In cannot. The diagnostics will have data about this.
Thanks for the tips, will try them - much appreciated!