The Pro Trading Room Alerts app requires that you log into the actual trading room via a web browser at least once every two weeks. If you're missing a room from the PTR Alerts app, try logging into the trading room and then recheck the app to see if it can be added again.
So if the room was there, but disappears, try logging into the live trading room. This probably doesn't need to be done at a time when the room is broadcasting, but if you try this after hours and the room doesn't appear, give it another try while the room is live.
Note that you can't use the PTR app to view the room to satisfy this requirement - you must log into the live trading rooms you want to access via a web browser at least every two weeks. This can be a browser on your smartphone.