I just got my first crossbow this year too for my grandson to hunt with. I sighted it in and couldn't believe how loud it is. Couple that with a whitetail that is a bundle of nerves and it is quite possible that the deer is ducking the shot. At the sound their first reaction is to drop their front end to recoil out of the area.
If you're aiming center of chest they could be ducking the bolt. Next attempt try aiming low (heart area). That should give you a couple more miliseconds to get a vital hit.
If you're looking for a different bow, I may sell the previously mentioned crossbow with all the goodies for a real good price. Its and Excalibur Ibex. Drop me a pm