I opened mine up and didn't find any kind of groove like suggested above, so the pressure plate idea seems reasonable. I wound the sprocket with my finger and couldn't find any rhyme or reason as to when it decides to click and stop rotating. Same thing with the winding post... sometimes it would stop, but most of the time it would just spin and spin.
So I'm wondering, if the sprocket just spins, then I don't know if the pressure plate is the culprit. But all bets are off when the back is open, so IDK. Approximately how many turns should it take to advance 1 picture? Maybe I'll load up a dummy roll and see if I get consistent rotations.
On a lighter note, I'm probably one of the few people shooting negative film in a stereo camera. I'm planning to make flicker gif's and also print some b&w pairs in the darkroom and mount them on stereoscope cards for viewing. Anyone done this?