A common issue for a plastic tooth zipper is slider damage:

A loose slider won't keep the teeth together effectively and they will split open after the slider has been pulled up (reference image #1).

A compacted slider will be too tight and it will be difficult to pull up the slider (reference image #2).

If either of these are the case, or if the slider is missing entirely, it's likely that a Plastic Tooth Zipper Slider Replacement will fix the issue.

First check that Steps 3 - 5 do not apply to the zipper and then proceed with the slider replacement.