In the last month or so I have found that the Android Volumio app does not find one or both of the two Volumio units I have on my network. If I start the unit locally through its own touchscreen, or if I can see one unit and I access the second via the multiroom function, then it also appears in the app.
Using a web browser works without issues.
I see the same problem on both my setup and that of my neighbour, she has only one Volumio unit. We both use AVM Fritz!Box routers, which might be a reason for the behaviour bbut I have not found a router reboot to help.