Yes, it would be a network issue, and not an issue with the app. 

Any chance you have a mesh network? Some meshes use different subnets for different ‘pucks’, you could see this behavior when a controller device is connected to a different subnet than the one the Sonos is on.

Another possibility, although less likely, is a duplicate IP address for your controller device. This is pretty unusual for a phone, though . You could try powering off (completely, not just ‘sleeping’) the phone, then reboot your router. Give the router a couple of minutes to come back up before turning the phone back on. 

I agree with Bruce above, this is ‘most likely’ a local network issue.

The discovery of the Sonos products by the Sonos controller App is a slightly complex area to explore, but briefly… 

SSDP (the UPnP simple service discovery protocol) is used to initially discover the Sonos players and is done via multicasting via UDP to 239.255.255.250:1900 and, for good measure, broadcasting to 255.255.255.255:1900. 

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.

Thanks for all the responses.  

I disconnected the range extender (even though the Move is on the the original not extended network). The speaker still sometimes appears in my device list, and sometimes not. Noticeably, when I switch it to bluetooth, it always appears; when I switch it back to wifi, the Move is not available.

Following your advice, I tried rebooting the phone.  This seems to work to get the Move to reappear. This seems to work reliably. This is a bit annoying, however.  

Also noticeably, this problem never occurs with the Beam on the same network.

If it’s an iPhone then there are perhaps a few things you can consider…

If using, disable…

• VPN Client 
• Firewall software
• Antivirus software
• WiFi Calling/Dial assist
• Private network address (MAC address spoofing) in the local iOS Network connection.
• Mobile data access for the Sonos App 

How are we this far into the thread without knowing anything about the OP’s local network hardware/configuration?

My money is on a router unable to route SSDP between 2.4 and 5GHz devices, which  is why one phone works sometimes and another at other times: as they roam they are likely to switch channels/frequencies.

Easy validation step is to temporarily disable 5GHz on the router and see if things improve.