I am having a peculiar problem with my Mamiya RB67 120 back. With a roll of Fuji Velvia loaded, i can wind to frame 1 and trip the shutter, but when I advance the frame the indicator stays on one no matter how many times try to wind on. It has happened with all four rolls I tried (two 50's and two 100's). Strangely enough, I have loaded and shot several rolls of Ilford film with absolutely no problems. So it seems my back has a problem with Velvia. I searched for info on this, and the closest I came to an answer is something about "film thickness syndrome." Can that be the case here? Is this something that can be fixed with some sort of cleaning?