It sounds like it may be hitting the floor and the safety is causing it to reverse to the open position. Find the 'down limit' on the side of the motor unit and with a small screwdriver turn the control just a bit in the opposite direction of the arrow printed on the cover. If the door reverses 1/3 of the way down you need to check the door sensors and you can also increase the 'down force' just a bit. That control is located on the back near where all those white wires go.