There are a couple of reasons this might happen. First make sure it closes by hand all the way, if not there is something wrong with the door that will need attention first. If it does close then hook the opener back up. On the opener rail at the end by your header there is a little magnetic relay that is wired to the motor head. This can be moved farther up or down the rail. This sensor tells the opener when to stop in the downward direction. Loosen the set screw and move it toward the header 1/2 inch or so and try to lower the door. Do not move it too much at one time and if you do the door will hit the ground and come back up. Thanks BB