[John Rogers]

What generally happens is the outside edges of the zipper teeth get worn very slightly and so does the inside of the slide portion of the zipper and eventually one day there is not enough pressing together of the teeth so they latch. You could try squeezing the slider at it narrow end just slightly so the teeth are pulled a bit more tightly together and do it on the outside and inside portions. Not much is needed and it is easy to crush it so care is needed. To replace the zipper on a case like that I imagine nearly the whole thing will require dissassembly as the zipper is sewn in with it laying flat.