I wonder if one speaker is not updated to the same firmware as the rest. Maybe. On the Windows app (or in the phone app), ‘Check for Updates’ and run an update if required.

I also wonder if a static/fixed IP address for this errant speaker will help, in the router?

Another option is to completely uninstall the Windows app and reinstall it. Sometimes with the Mac and Windows apps, the app itself will update but not fully install, so a clean install can be a better option. 

 Hi Rhonny, 

All the speakers are on the same firmware and same Version of windows app, IP addresses are fixed and they connect there own WIFI AP.

Is it a laptop? Sounds like the router has trouble when a device flips frequencies between 2.4 and 5GHz (either the speaker or the device running the app). I’ve never heard of any issues with the Windows app itself in this regard, but many routers can’t handle multicasts between radios.

Not sure, Its a mixture of laptops and desktops and this the only speaker in the building that's giving this issue