Couple of suggestions that may help. You need pliers for one. Make sure slider is all the way DOWN. Gripping the top & bottom of the slider with the pliers at the edges of one side (you will have to do this to BOTH sides) squeeze gradually & GENTLY. You should see the top & bottom edges of the slider moving closer together over the teeth. DO same to other side. If you squeeze too hard, the slider will lock - you can ease the edges apart again, but you risk breaking the slider if you aren't careful enough. Failing that, it is possible to simply replace the slider (not the whole zipper) - check your area for tailors, dry cleaners, etc. It's cheaper than replacing the whole zip & only takes a few minutes.