My Sears garage door opener (only 2 yr old) needs an adjustment - the door only closes half way down, then stops. When in a closed position, the door opens all the way up - and seems to go too far in fact. The door works fine manually. There was a problem with the chain coming off the sprocket; that's been fixed. Nothing seems to be broken. Nothing is obstructing the safety beam. I've tried adjusting the + and - adjustments at the back of the unit and the open and close adjustments on the side, but this isn't doing the trick. I've misplaced my manual. Does anyone have any suggestions?