Devices missing from Things List

I have found that I have ‘disappearing’ devices and rooms.
Looking in the Android app, not all devices/rooms will be listed.
If I reselect my “location” within the app, it will usually find the
missing devices. Sometimes I have to reselect my ‘location’ more
than once before the devices are found (I only have one location).

Sometimes the ‘missing’ devices continue to operate as desired
(turn on due to motion, respond to Amazon Echo) and sometimes
they do not.

Any suggestions are appreciated ?