I have an RB67 Pro-S body which does not always trigger the motor advance back to advance the film, and so I have to manually trigger the film advance periodically. Is this a problem with the back, or the body? I suspect the body, as it certainly seems much older than the motor wind back. If this is a body problem, does anyone know the level of effort/cost required to repair it? Or would I be better off just buying another RB-67 body?