It can't drop by that much when closed, are you sure you do not mean when the door is opened it drops down off the catch?

If so, then it is wear in the hinge nylon inserts where the pins sit. You can adjust the hinges up to compensate a little, but replacement is the only permanent cure, which is a right pain as it is welded to the door on one side and bolted on the other.

Caused by people putting weight on the door when opened, i.e. leaning on the top of it, etc...