Inspect the rubber o-ring on the non-functional wheel. If there is cracking, a flat spot, or damage to the o-ring, it will need to be replaced. New o-rings can be purchased from www.quilttech.com.
The best way to replace the o-ring on the encoder is to remove the encoder wheel by loosening the setscrew. The new o-ring can be pushed on with the help of a table surface.
If this is not the problem, switch the position of the two sensors on the board. If the same sensor doesn’t work, then the sensor is faulty. If the other sensor doesn’t work, the cable is faulty. Contact your dealer for a new sensor or cable.