Hello,
Get some petroleum / vaseline and put it inside of the zipper head (and on the teeth if possible, not sure how accessible the teeth are as I do not not own this bag but wanted to help out). This is a common maintenance process when using a cooler I have, this zipper is almost impossible to use with out it (different brand, comes with a little bottle of petroleum). Additionally, you can add a clip of some sort to gain more leverage when pulling. Hope this helps.
-GK