If i was you i'd just change my aim point a little. Heres the rule of thumb i follow. If its nerveous and 20yd or closer i just hold on the heart if it ducks the string ill still catch the lungs. From 21-30 if its really nerveous ill hold right on the bottom line of the deer directly below the heart if he ducks i still catch lungs. I dont shoot past 30yd so i can't tell you how to aim past 30. It took me 2 times or trial and error by missing with them ducking for me to figure out where to aim. It doesn't matter how fast or how quiet you're crossbow is if they are nervous they are going to duck. So just aim a little lower and good luck!