I would assume that if it has come off before then some of the clips are broken, which happens easily if they are incorrectly removed or refitted. Mine were loose when I purchased the Zed due to broken clips and as they were slightly scratched invested a few £'s in new ones.
Check carefully but it should have one clip at the lower edge that locks into a slot in the handle, then a series of moulded clips all the way to the top on each side that clip in. Once locked there is no way it will come out unless damaged.
