Using a small flathead screwdriver, insert it into the female large end of the zipper jaw on the high end. The high end being the end where there are teeth exposed, the low end being the side of the zipper which has already reached its end.
Twist the screwdriver so that it opens the zipper jaw a touch and, at the same time, try to pull the zipper tab down to meet the other side- that 2-3 tooth difference