Sounds like you may be experiencing a melt down---that is emotionally speaking of course. The excitement of the moment causes your brain to turn to mush and all of things you worked on in practice gets flushed right down the tubes.

All that has been mentioned is certainly good and should be taken into account. Practicing with your hunting clothing on, aiming at the LOWER third of the deer in case of a string jump, solid and consistant anchor, and practicing with broadheads out of an elevated position are important. Not paying attention to any one of these could spell M-I-S-S.

I have also witnessed people who have done all of the above yet when a animal steps in front of them they lose their ability to focus and just point and shoot. Certainly one can score the occasional hit, but sooner or later your going to whiff if you don't focus on picking a spot and executing all of the steps necessary to getting to full draw.

Have a "game plan" made out in your head as to what you are going to do (and when) once an animal shows up. Know the yardage that the animal is standing at and then pick a spot or hair on it and follow through as you let the shot go. It is important not to RUSH! Don't dawdle, but don't hurry a shot. It only takes a couple of seconds to do it right. Trying to rush a shot will only compound your problem.

It all sounds easy doesn't it. It will seem easy once you pull it off a time or two. But it will be your preparation both mentally and physically that will spell success or failure in the end.