Chamberlain garaged door will only close when button is held down; a simply press/release does not work.?

Unit is 1/2 horsepower.... To close the door; must press and hold depressed the button on the remote. Simply pressing and releasing door travels a short distance, stops, and returns to full open. Once door is closed; single press/release and door opens normally/fully. Checked sensors at base on each side of door; each has green light on when operating. Blocking once sensor green light goes off on nearest sensor; blocking other sensor; green light stays on. Track is lubricated and no wheels are seen to be binding; fact that the door does close fully when button is held depressed indicates not to be a door/track/wheel binding issue.... Not sure if sensor could be cause..... how to bypass not known...