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From the description of your problem, it sounds like your zipper slide needs to be tweaked or replaced. Squeezing your slide has been described above. If that doesn't correct the problem, you need to replace your slide. First you will have to identify it by size and manufacturer. There may be some info on the zipper pull. I assume the zipper in the leather jacket is metal. You may still be able to buy a new slide. Look at the top of your zipper for zipper stops. If you remove the zipper stops or whatever stops your slide from traveling off the zipper, the slide can be removed. Install another slide and install new zipper stops. Use needle nose pliers to crimp them on.
There is a good web site with zipper info that may be of help to you. There are also some You Tube videos on repairing zippers. If you can't buy a new slide, you'll have to locate a donor garment. The frustrating part of zipper repair is acquiring the parts. Once you work your way through the repair you'll be surprised how easy it was. Hope this helps. http://qualityzipper.com/