If you have a treadle, the measure eye is not seeing the reflector plate when dumping.
If you have a Virtual Treadle check your right side ultrasound sensor and make sure its working.
Easy way to do this is to just sit between them and listen to make sure both are clicking.
If not, replace. If it is clicking turn dial in vt to position 1 click cw from home (where the led's scroll) and see how many leds are light up, each led is about 17 inches of distance to what ever target you have in front of sensor.
hope this helps
Mike