Couple of suggestions. First, reboot (power cycle) both the Sub, and the speaker it is bonded to. If that doesn’t work, submit a system diagnostic, 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 Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.