This is most likely happening if you imported SongSelect songs into OnSong from somewhere other than the SongSelect integration within OnSong itself (In OnSong > Tap Songs > All Songs > "+" or "Box/Arrow" Sign > Import/SongSelect). You can only transpose/capo SongSelect content if you import it into OnSong from this described place. Unfortunately, this is a limitation of SongSelect's content currently.
This could also be happening if you do not have a stable WiFi connection. OnSong requires an internet connection to transpose SongSelect files, so you'll want to make sure that you are connected to WiFi and that your WiFi connection is connected to the internet.
This could also be happening if you do not have a stable WiFi connection. OnSong requires an internet connection to transpose SongSelect files, so you'll want to make sure that you are connected to WiFi and that your WiFi connection is connected to the internet.