When this happens to me it's usually down to a screw I had not noticed.
If you can separate the two halves slightly with a small screwdriver then the place where they remain close will point you towards where the rogue screw(s) is.
Miles Tools & Machinery site has exploded diagrams of some Qualcast Trimmers - might help you.
mtmc.co.uk/Spare-Parts
Good Luck..., October 2015