The door locks, sometimes, and not others, and if it doesn't lock then the security "beep" isn't heard. If I remove the little rubber cap and turn the white plastic cam with the key, then it locks and beeps and does everything it's supposed to do, UNTIL the next time the door is opened, and then again it doesn't lock and doesn't beep. I imagine there's something worn out or broke in the lock assembly. Any ideas?