I had the same problem on my back window.
The zipper on my back window was almost impossible to move. Once zipped, the teeth easily came apart. Drove me crazy!
I compared zippers (since there are two on the back window) and I noticed the bad one was slightly wider where the teeth go in. I got a pair of pliers and crunched the opening so it was narrower, and bingo, works great.
4.0 5-speed manual
Dana 44 rear
4 wheel disc brakes
31 x 10.5 Dayton Timberline A/T, with 15 inch Canyons