It can be a variety of things…

The ‘discovery’ of the Sonos products by the Sonos controller App can sometimes be a slightly difficult area to explore, but briefly… 

The Sonos controllers send multicast ‘discovery’ packets sent between the controller App and the Sonos players.

Both the Sonos players and controllers must share the same network subnet. However if the Sonos devices are connecting to multiple access points, the recommendations (from myself) are that they be set to the same WiFi channel and channel-width. This is for optimum performance particularly in groups and for some reason with some (not all) mesh-based WiFi systems, it appears some will work okay with Sonos, but others may not, particularly if the satellites/AP’s are perhaps auto-selecting their own WiFi channels. I’ve seen some people report issues with Google ‘nest’ mesh WiFi, for example, although others will occasionally say it works for their ‘small’ Sonos setup.

What is certainly not supported though by Sonos, are wireless range extenders, as some can mangle device MAC addresses and EoP (powerline) adapters.. so if you are using those type of wireless access points, to extend the wireless network coverage, then that may cause such intermittent device discovery issues by the Sonos Controller App.

Reserving the IP addresses in the routers DHCP reservation table can often help with device discovery, so consider making your Sonos IP addresses static. The router user-manual will normally assist you to do that in most cases.

Just to add aswell, it can sometimes be Apps/software running on the mobile ‘controller’ device that interfere with ‘device discovery’ Examples to perhaps consider are…

• VPN Client 
• Firewall software
• Antivirus software
• WiFi Calling/Dial assist
• Private network address (MAC address spoofing)
• Mobile data enabled for the Sonos App  

So consider disabling these features, even temporarily, just to see if that may fix your device ‘discovery’ issue.