I have had that problem. Usually due to when the get wet with the salt water over a period of time and get slightly rusty. I took a small thin screwdriver and put it into the opening of one of the tabs that you use to zip/unzip the bag. I pulled it open just an inch or two.l I then grabbed each side of the open zipper and just pulled outward with my hands. Not sure if will work for you but cannot hurt to try.
Last edited by JoeDev; Apr 23, 2013 at 06:53 PM.
Instead of pulling on the slider/pull tab, try pushing the teeth back into the slider. Grip the teeth as close as you can to the slider and wiggle them back and forth, one side at a time while holding the slider still. If there's corrosion maybe hot water or vinegar would work.