Usually the first part of a zipper to wear out is the slider. The metal wears away over time and no longer pushes the zipper teeth together. Replacing the slider is often a simple matter and takes only a blade or seam ripper and a needle and thread. If you have an Oware product with a worn out slider, contact me and I will send you a new slider for free. Here is a couple of videos on replacing a slider on a bug bivy.
Zip slider replacement a part 2
Oware 