the International Rollei Club

I have been using the SL66-E for many years and recently bought a new back, used. The film transport is off as evidenced by a large portion of the film unexposed and the 12 images compressed with overlap in the remaining film. The exposure overlap is about a quarter of an inch.
I suspect there is a problem with the film back because the film does not "lock" soon enough when turning the pop-out knob on the film back. There is some difficulty getting the slide to go into the back when empty or loaded with film, but it does go in completely. Can anyone help me diagnose this problem, and suggest a fix? I suspect that the back is the problem, but need to shoot with another back to confirm this. Thank you!

Well, I think my problem is solved. The small gear on the back of the film magazine was not in correct position. With the slide out, I needed to rotate that gear in the direction of the arrow until it locked. Now the film slide goes in without resistance and the knob the knob on the side of the magazine locks properly to position film at the first exposure.