Basically, the zipper pull has become loose over the teeth and is sliding down over the teeth too easily. There's nothing wrong with the teeth, it's all with the zipper pull.
Take a clamping tool (pliers work well) and apply it to one side of the zipper pull, while the zipper is open (on either side of the pull tab) that actually goes over the teeth and clamp down a little bit. Then do the same to the other side. It doesn't really take a lot of pressure. Then test. If it holds better, then you are done. If it doesn't hold, apply a little bit more pressure. Don't do too much or you'll never get the pull to zip and unzip. Then you're screwed.
Kate, August 2006