Its possible the sensor has gone bad

you will need to UNplug the unit , then take it apart 9 screws then gently pry it apart with a screwdriver or butter knife
you could test this with a voltmeter, one side emits a constant should have 5-12 volts across this
the other side the reciever will change its voltage when you break the signal with a piece of paper.

also ive seen chunks of plastic break off from inside and as the teeth rotate the plastic can cut the wire