In Zelda: Ocarina of Time, how do i deflect deku nuts off my shield inside the deku tree?

I just got in the first dungeon in the game. i'm up against the deku scrub, and my shield isn't deflecting, even though I'm locked on and holding down the attack button. On the game, it appears that my shield is out and that it will deflect them, but i just keep getting hit. Help!!