The turning effort is not the same for the full rotation, and the last part is a little tighter, as Film_man suggests, and that is normal behavior. But it shouldn't be much different than what you may notice with the a24 back. If the camera seems smoother with the 24 back, the 12 back may need some servicing.
Have you processed the film from the 12 back, if so, is the frame spacing ok? irregular spacing would be another indication the back should be services.
As for 120 in a 24 back I think 10 or 10.5 frames would be the expectation because of the thickness difference. 120 has backing paper along the whole length, but 220 only has paper at the beginning and end.