Rollei SL66 Film Back-Improper Film Advance

I have a used Rollei SL66 120/220 film back which has a film advance problem. When in use the film back has a moderate degree of extra resistance on film winding at the last 5-10% of the film crank. Even with extra force to complete the film crank, the film does not advance enough and there is a resulting double-exposure, overlap of shots, of 4-6 mm between shots. This appears to get worse as the film is used up, being most extreme with shots beyond 8 or 9. I suspect its related to a tension adjustment, but cannot see how to fix the problem. It is not the camera, since another film back works properly. Also the inner mechanism and the film back housing are a matched pair based on the serial numbers. Any suggestions as to how to alleviate this problem would be appreciated.

I assume you bought the wrong thing or didn't check properly before buying. I own a Rollei 66E with two backs. Your problem seem to me, that the mechanics inside the back is broken perhaps some gears. Filmadvance should work smothly and with constant tension. Send your back to a repairshop of Rolleis to check or/and fix it or give it back the the one who sold it.