This is a problem area with the Bachmann 0-6-0 and the 2-6-2. I own six of these locomotives and have lots of run time on them. I've seen this failure once before. You seem to be particularly unlucky with your two. I'd give the service department another try. Bachmann has the main drivers listed in the parts section for the 0-6-0 but they have been out of stock for some time. Maybe the service department can find one or two for you.
Have fun, Mark
Have fun, Mark