In reply to The Lemming:
If its a double slidered zip, when you do it up make sure the bottom slider is all the way down, and the top slider on to it,
hold the fabric to the side of the sliders not the sliders when you locate the other side of the zip, keep holding the side you just pushed in and pull slider up with other hand
Reason I say this is because the jam can be caused by the zip not being located correctly at the start, it goes up but wont go down, if you start right it will undo
common problem on a lot of double slidered zips
HTH