Sometimes Spotlight's index gets corrupted. When this happens, it won't find certain files or folders. You'll often discover this when you *know* a file exists somewhere but a Spotlight search doesn't uncover it. Here's what I recommend: Have Spotlight reindex the entire computer. Do it at night before you go to bed -- it'll be done when you wake up. Here's the command line way to do it:
Step 1: Open Terminal.app
Step 2: Type in: sudo mdutil -E /
Step 3: You'll be prompted for the admin password. Enter it.
(thanks to enckwanzer)