There are several reasons why cheats don't work in Dolphin. The most common cases I've seen are:
1: The cheats are for the wrong revision. This usually results in crashing, or odd effects. Sometimes it does nothing as well, but, they can be catastrophic
2: Dolphin's cheat handler does something wrong. See: Wind Waker MegaJump code.
3: Some Gecko Codes simply don't work in master. I don't think this affects GC games though.
1: The cheats are for the wrong revision. This usually results in crashing, or odd effects. Sometimes it does nothing as well, but, they can be catastrophic
2: Dolphin's cheat handler does something wrong. See: Wind Waker MegaJump code.
3: Some Gecko Codes simply don't work in master. I don't think this affects GC games though.