Also do not apply what I call Gorilla Torque. Do not overdo the tighting snug is fine. The screw will not fall out.
Most of the time for me is the loose or poor solder joint. The capicitor will fail and the loco will not reverse. It is the capicitor on the top right edge of the board. The lead is not in contact with the solder. And you must take a very close look(magnifying glass).