The return spring is broken inside the switchbox. Have the switch box repaired. The pad or its driver rubes against something and when the machines turned off it stops immediately. Turn the pad by hand and visually check whether it contacts the dust seal. If it does, the pad is larger than 20 inches or became eccentric.
Some pads grow more readily than others.
If the pad does not contact with the dust-seal then take it off. If a scraping noise is heard, the motor mount may be out of alignment. To correct it, remove motor cover and measure the distance of the rubber bumpers from the base. This will eliminate the pad contacting with the dust-seal but will limit the thickness of the pad you may use in the future. We allowed the motor frame to retract higher by letting the mechanism go further back.