It doesn't seem to be about pressure, but how long you hold your stylus down. When playing on a real DS, it's a fast tap, not a light tap. I don't know why the window to release your stylus seems so much shorter on the emulator, but I found out that it's a lot easier to unstack items after slowing the game to 50%. Sorry about posting on such an old topic, but I had to figure this out today and thought I'd share.