That's an issue with the zipper slider itself and I have experience fixing it. It's difficult to maneuver the tool because of fabric in the way and it's in a generally awkward spot but possible. Take needle pliers and squeeze down *a little bit* on the back of the zipper slider itself. Doesn't need too much of an adjustment, so squeeze a little and check, and squeeze a little and check. It's the small end of the zipper slider.