Not all inserts are compatable with all shells. Swapping inserts can cause another line of problems.
That gear you see might be one problem but not generally the cause if it is in good shape on visual inspection. The internal gear that mates with it should also be inspected for damage.
The waffer inside the the counter mechanism is most likely the cause in 9 out of 10 back failures not advancing film properly.
*Try another back (don't swap insrets) n see if it has the same symptoms, this way you'll eliminate the body to back interface gears?
As you say the other back works, the one that is slippping is a junker, throw it out n get another to save you alot of grief.
If you have a small screw driver handy, peel off the leatherete n open the side with the counter n see it work for yourself? Notice the counter wheel has slots, that is the waffer gear I am telling you about. Once it is stripped or damaged, it is junk n can't be replaced.