I'm going to presume you have a special, post 76. Lots of assumptions here but, that's what we're here for.
flip the lid open stick the key in and turn it. the key moves a plate. the plate should move freely back and then forward again when the key is released.
The plate can be bent or damaged if the lock is ever forced open.
I would suggest hitting the underside of the latch with some lubricating oil - not wd40, but 3 in 1, or pb blaster.
Make sure the latch moves freely with the key and then moves back into place when the key is released.