The first thing to try is to do the stereo setup routine, as described in step D-3 of the troubleshooting guide. If that doesn’t fix it then make sure the magnet is placed in the correct orientation, making sure the sticky tape is placed on the side with the black mark. You can also try to take out the sensor board (with the pot block) and use a different magnet to change the volume…move it closer and further away from the sensor as you listen to the volume. Remember the polarity of the magnet matters. That test will at least show you if your magnet is backwards and if the sensor is working properly. Other than those 2 things, I’m not sure what to tell you. I’ve never had this issue before. Hopefully the stereo setup will work.
Good luck!
-Brach