If the PCB has front mounted controls which poke out through holes underneath the screen at the front, check that they are actually all lined up with the holes and poking out through the holes, and DO NOT apply any more force otherwise you run the risk of breaking whatever it is that is preventing the PCB and rear cover from fitting fully home. Also check that everything which should protrude out through holes in the rear casing (adjustment controls, connection sockets) is also lined up with the holes in the casing.
When everything is lined up OK the cover will just slip on as far as it should go without any problem.