To unsnag your zipper, first try pulling through to the end and, if it can make it, reversing the zipper. That should release the snagged lining. If the zipper has a decent amount of lining caught, use your pointer finger and thumb to roll back the lining while you zipper forward incrementally. The break in pressure could encourage the zipper to release the lining, but this method takes some work. Some users also swear by applying Windex, WD-40, or clear lip balm to a jammed zipper.