I finally popped and bought a GS-1 system. Its condition is very nice but as a newbie, I'm having a winder issue that I've not been able to solve.

When loaded with film, I'm able to fire off one or two frames, then the mirror stays up (naturally!) but the winder is stuck in position at that point and won't cycle. If I remove the film back, the winder and shutter cocking all seem to work fine again for 1-2 frames. Troubleshooting had me try the following without success:

1. Same behavior with multiple film backs.
2. Replaced battery with a fresh cell.
3. Same results with and without the speed grip winder.
4. Confirmed correct film loading.
5. Tested with different lenses attached.
6. Mirror lock up and multiple exposure levers are correctly positioned.
7. The film back(s) seem to be turning freely and not binding.

The seller had not experienced this so I'm wondering if I'm doing something wrong and not realizing it. The system is very complete with 5 lenses, different finders, 5 film backs and cost quite a bit to ship from Japan so I'm pretty much committed to sorting it out on my end. Any ideas or have you had this issue before?