I work for dry cleaners and see this a lot. When they press the wet jeans they have to put a LOT of pressure on them. Sometimes they can smash the zipper pull. You may want to CAREFULLY,using a small flat bladed screwdriver, pry the zipper pull apart a little until it moves again (I don't get a lot of success with this) or ask the dry cleaner to replace the zipper.
Chris
March 2010