The SONOS controllers are impatient. If there is an issue that delays discovery, the controller will give up. It’s possible that your PC sometimes has a lagging network connection or is distracted by another program. Is your PC wired or wireless?

You can find the IP addresses of the players in the Android App. Go to Settings → System → About My System. What is the IP address of the Associated Player? Are the players indicating WM: 0 or WM: 1? Now you can go to a CMD prompt in Windows and PING each IP address. Are the players reachable in a reasonable time frame? 

How does each player connect to the network? Wired? Wireless? Which model router? Network switches? Wireless mesh? SONOS models?

Thanks @buzz 

I have to confess I got it wrong in my original post. 

All the rooms were not there in the Android app. Turns out the Sonos device on the Windows app was on a different IP address range from the rest.

Now that I have fixed that all my Sonos devices really are there on my Android app but now I’ve none available on the Windows app.

That’s because…

I don’t know if it’s possible that I can connect to my Sonos system via my Windows PC?  My PC runs a network bridge connecting to both the wired network (192.168.0.xxx) and the wireless network (192.168.152.xxx). 

The Sonos system connects via the wireless network.  Is there anyway I can make the Sonos app on my Windows PC ‘see’ the wireless network?  It has access (via the bridge) but when the app searches it doesn’t find it?

Please don’t ask me to explain why it’s set up this way -  I don’t actually know and because I’m a bit out of my depth I’m loath to make changes in case I break the setup for other components!

Other than reboot my PC, I didn’t do anything else and the Sonos app on my PC is now working again!

The SONOS controllers and players need to be on the same subnet.