Hi

I just buyed a used Mamiya 645 AFD with a film back and a 80mm lens.
I made a test shoot with 2 rolls of Fuji 400H 120 film. I ordered contact sheets and I found a problem: The camera winds film loosing the first 3 frames in both rolls. Then it begins taking pictures starting with "1" in the film counter. So...when I make the "12" picture... the film just finishes but the camera allows me to take 16 pictures... I just lost the final 4.
In the attachment you can see a yellow rectangle showing the lost 3 first frames and a green rectangle showing the last picture... just cut because the roll finished there.
Any ideas to solve this problem? The camera is supposed to have a film sensor which detect the beginning of the film when the back is inserted and the first winding starts. Is it failing?

thanks

Ernesto