I suspect that this is a duplicate IP address issue.  Please see my post here: Tips & Tricks - Resolving random issues impacting Sonos devices.. | Sonos Community

Thanks for your help. I’ll have a look in to it, and hopefully your work arounds will sort the issue out. 

So I’ve had a look at the IP address situation, and followed your advice. Unfortunately, after working fine for about 12 hours, the issue has cropped up again, only now, even my Beam (which is connected to the ethernet) is missing from the app. As mentioned previously, all speakers are showing up in the app on the iPad, but not on the phone. 

Do you have a BT Smarthub by chance?  If so, have a look at this post: 

Aha!!! Yes, I do. Thanks, will give that a shot. Thanks again for your help!

Thanks again, this seems to have sported the problem (fingers crossed). I’ve logged the issue with BT too 

Encountering similar issue with one app controller device (phone). Sonos system is online and operational, all speakers working and connected. Accessible through other app controller devices. The subject device shows a few of the speakers but not all. Have removed system, cleared and reset app, reset speakers and local wifi. Using home network not Sonos net. Other Android device apps working normally and connectivity is good. 

It's not a connection or wireless network issue it's device specific (Android phone). Other Android device app controllers work as expected.  Any suggestions? 

Sorry for a basic suggestion but have you booted your android phone?

Yes I have should have clarified. And cleared and reinstalled the app on the Android device as well. All to no avail. It's an odd scenario and I consider myself savvy with tech from both hardware and software perspective. It's a head scratcher and I've been researching from different angles

Are all your Android devices connected to the same wi-fi both name and frequency?

Yes that's correct same wifi same frequency, 5ghz in this case. Running both 2.4ghz and 5ghz. All Android devices same connectivity. Removed (forgot) network on device and reconnect. Used the Sonos app troubleshoot to find lost products which revealed the missing speakers but cannot rectify, never added despite repeated attempts

Can you force your phone on to 2.4 connection which is what most Sonos devices are connected to.  What devices can you not see?

Appreciate suggestion although didn't seem to be the cause. I believe it's resolved through a series of device and network reset sequences.

Even though the Sonos system was operational and accessible on other Android controllers it must have been some sort of IP conflict or failed network traffic handling. Strange set of circumstances. Wish I knew more definitively what the solution was to share with others, and for my own sanity