How to repair a zip?

[Pressgang1]

I know the 'pencil' trick for sticky zips and I know how to repair one if the slider becomes detached but my problem is this: the zip, when closed, comes apart around half-way up. It's as if the teeth aren't gripping perfectly and I end up with the slider at the top, the catch at the bottom, and this huge gap in the middle - not a good look :o)

Any ideas how I can 'tighten' the zip?

[grasscarp]

I have done a lot of zip repairs in my time. This one sounds like it is too far gone and needs a zip replacement. If you want to do it yourself the trick to putting a zip in, is to do the sewing with it done up. If not, some cleaners do this kind of repair job.