[KOUKI KA-T]

Hmmm, I had that problem once when my car had been mildly hit in the rear.
I had to flip over that small metal tab on the lock to make it close (looks like that's what you were doing as well) the trunk would randomly open on its own sometimes though. That worked for me for about 2 weeks until I replaced the lock. I've got 2 extra trunk lock mechanisms from some trunks I have lying around but no keys to them, you can just have one if you can't fix yours.
-Sean

EDIT: Also try messing with the latch hook itself, you could try loosening it and bring the trunk down enough to kind of move the latch into a better position and retighten it, maybe it got shifted somehow.