My garage door closer will not shut all the way. It goes down almost to the foor and then goes back up. When I stop it, it blinks about ten times. I have ensured the sensors are lined up for the entire time by obserivng the indicator ligh and it stays bright and solid green, I have adjusted the down pressure setting, I have manually opened and closed the door. It will close and stay shut if I hold the button down until it reaches the floor. Any ideas? I am frustrated