I've done a fair amount of searching, without any luck. I'm hoping someone here will have a suggestion.

I have a Bronica ETRSi with two 120 backs. One of the backs tends to have problems mid-roll. Usually what happens is that the film simply won't advance. It feels like it does, but it doesn't. The winder winds and feels normal, but the frame indicator doesn't advance, and I end up with multiple exposures. Eventually, it will "kick in" and start advancing again.

The shutter works fine, the dark slide is out, the multi-exposure switch is set to single exposure. I haven't had problems with the other back, so I'm assuming it's a back issue. I'm reluctant to just buy a new back and assume its fixed, because it kind of stinks when it freezes up when I'm out and about.

Any suggestions for things I can check/do?