Type out the passphrase in notepad or some other editor to check if it really comes out the way you think it does. Capslock and accidental keyboard-layout-swaps can cause this type of confusion.

Did you successfully unlock the wallet with this passphrase before? In other words: Are you sure you used the passphrase you intended when you first encrypted the wallet? Writing it down on a piece of paper won't help much if you accidentally entered it with capslock enabled.