Stuck clambshell release button/latch

When you close the PB, a small hook-like thing comes down, and locks the clamshell (Though this isn't easilly visible) chances are, that that hook is stuck. You could use a piece of tape to prevent the hook from locking the clamshell, but it wouldn't stay tightly closed. It is unclear to me how the hook descends (whether it be gravity or magnetism) but it may be stuck in place by the latch mechanism, or may be slightly bent. You might want to try "Catching" it as you close the lid, and inspect it for damage.