Man, that's a hard one to fix. Sounds like the polyester in the zipper tape shrunk a bit because of the heat. Hard to say how to fix because they get all twisted. If you have pics, feel free to send them over and we can try to offer some solutions.
Trying to fix one tooth on a plastic zipper won't work out so well. The teeth are pressed on at a high temperature and once a plastic tooth is damaged, it's highly difficult to return your jacket back to original quality or use.
