there might be something else going on. your latch is held in there by two screws (torx), and if they were not tight enough, everytime as you shut the trunk, the hook part of the trunk would slam into the latch. this could push the latch downwards with several milimiters. what will happen eventually is that the latch will slide downards and when you close the trunk, its hook will not go down far enough to push on the latch's lock mechanism, so it won't lock... if that makes any sense...
so the fix is easy. loosen the latch just a little but, pull it upwards a few mm's, and tighten the screws...close the trunk several times to see if it will lock.