On my mobile app:
When I select 'devices' it only shows one location for one Roku Express, however I have 3 other Roku devices set up. Why can't I access the other devices?
The Roku app on your phone should find all Roku devices connected to the same network as the phone.
When you go to the tvs with the "missing" Rokus, do those Rokus show as being connected to the net? The same net? Settings > Network > About