I recently acquired a Mamiya 645 Pro and sometimes after taking a shot I cannot advance the film. At that point I have to take the film back off, put it back on, and then I can wind the film again. The first shot after doing so is not advanced and I end up with a double exposure. It seems pretty random - not like it always happens at a certain frame but seems to get a bit worse maybe the further I get into a roll. This is with a 120 insert and it does go to 15 before winding out. Things I've already tried: - cleaning the 4 contacts with an eraser. - cleaning the pin with an eraser. What do you think the issue is, the film back itself or the insert? Or, something else? Thanks in advance!