Were you on the ground? If so, was it a high hit that clipped the top of the lungs? This could account for little to no blood as it may have been draining into the chest cavity. If you were up in a tree, the exit wound would have been on the lower part of the chest cavity, allowing the blood to drain out. Did you check a wider path than right under the deer's path. If she went down in 30 yards, you probably got an artery and blood may have spurted way out, although you would think you would be able to see some near her path. Just some thoughts from an inexperienced tracker.

The most important bloodtrail leads right to the foot of the cross