My 1/2 hp craftsman garage opener won't go down

It sounds like it is out of alignment. The motor does detect excess pressure incase something or someone is underneath and the light sensor is not detecting anything. The pressure on the motor does change from when the door is all the way up to when it has the full weight of the door on the chain before it touches down. The spring offsets most of the weight of the door, but it still takes weight. Maybe when the motor is taking more of the weight, it is still shifting in a way to throw off the alignment. Also, you could disconnect the door from the chain, try manually moving the door and see how much weight the motor is taking at what points. Good luck!