There is a known bug with the Android app.
How to fix
- Kill the app and restart it.
- Turn the airplane off and back on.
*The Android app will be updated by the end of January 2018.
General rudder check
- Verify that when you tilt the phone the horizon in the app also moves (this shows that your smartphone's gyro is functioning).
- Remove the rudder and verify that the area is clear of dirt and plastic residue.
- Verify that the rudder has a magnet inside (small round shiny metal piece).