If your door will only go down when you hold the button on the wall, it could be something blocking the sensors. Often times, the safety eye sensors will get caked with dirt or grime. When that happens, you need to wipe it off completely.
Another issue could be that there is a faulty wire on the sensor. If you have cleaned it and still need to push the button down to shut the door, you may need to have the wire changed. Check to see if it is lose or if there is any damage to it.