Minimize, Moonblast, and such are charge attacks. That means you need to hold down the A button until you begin glowing. On release the attack will come out.
Note: Minimize's purpose is to be used as a dodge and can be cancelled with a Y or A attack.
The reason you kept using Will-O-Wisp is because you have to press the A button to use Will-O-Wisp, but for Minimize, you have to press and hold the A button, so the game thought you just pushed the A button because you didn't hold it long enough.