Are you moving around a lot, such as strafing? If you watch other players move on your screen, there is no such movement as a strafe, it's simply a client side graphic that looks much better than the reality of it. If you strafe to the right of the mob as it's dying, you're facing to the right of the mob, away from anything that you may be trying to position yourself to hit. You need to be standing in place for ~1ish seconds in order to auto attack anything within your cone of vision.