GTA IV Auto-Aim Problems!!!!!?

Okk so I have been having really weird auto-aim problems with GTA IV. In single player, and in multiplayer, I can't seem to get the guns to auto-aim. I can lock-on when I have just my fists or the knife out, but whenever I pull out a gun, I have to free-aim. I know that your supposed to press the trigger all the way and thats what I'm doing but I can't get it to auto-aim. hellpp!!!!