Problem with Mamiya 645 AFD winding film

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

Are you advancing the film manually to the "start" mark before closing the back?

If so, you aren't doing the right thing, because the camera does that for you once the dark slide is removed.

From the manual: "gently insert the leader paper tip into the groove on the empty spool that is on the “SPL” side, and turn the
spool one or two turns to wrap the leader tape around it."

No. I don´t advance to the Start mark. Just two turns until I feel the film is secure in the spool.

netog said:

No. I don´t advance to the Start mark. Just two turns until I feel the film is secure in the spool.

Click to expand...

That's not right either. There should be an arrow on the insert to line up the arrow marks on the backing paper.

tkamiya said:

That's not right either. There should be an arrow on the insert to line up the arrow marks on the backing paper.

Click to expand...

Not for the AFD - it is supposed to speed up loading by sensing the beginning of the film automatically.

Any ideas to solve this problem?