Zippers are great until they break.
There's always the chance that even the best quality zipper could separate, and develop a gap behind the slider. But before you throw away the item, or rip the whole zipper out and replace it, try this simple repair method. Look underneath the zipper to see if any threads are caught in it. If there are, take a scissors or razor blade and cut them where they enter the slider. Pull them through to free the slider.