Hi there!
Have you possibly checked the battery in your key fob itself? Usually dealerships are able to test and see if the batteries are actually good or bad. Silly, I know, but it's most certainly easiest to start with the cheaper alternatives. It would be best to dive into things that are central to your locking mechanisms in the vehicle.
Try searching around our tech articles here:
BMW Technical Articles - E30 E36 E46 E90 E60 E39 Z3 - Pelican Parts
Maybe you'll find something relating to your particular issue. If you do not find something I am sure that one of our technicians may better assist you in finding out what may be wrong.