You don't have to drag down. You just have to drag parallel to your character.
For Example, if my character is this 0. The lines represent different ways I can perform a side slash:
_
l0l
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Sorry the small bottom line didn't work, but you get the idea. You just have to drag the stylus parallel to the character as opposed to directly away from him. At least that's the way I understood it, and it works every time so that's all that matters lol.
Btw, I have now completed the first full dungeon. It was fairly easy but lots of fun. I just wish that damn fairy would shut up. *has Navi flashbacks*