Well that is odd. A lung shot without a trace of blood? How long did you wait after shooting to trail it? I am guessing you may have been a little high of the lungs - too high for the lungs and too low for the spine. You then hit nothing to bring them down and little blood is lost, and they can even heal up and live. This can happen from the ground, although from a tree stand it is less likely because of the angle. See
http://www.fieldandstream.com/answer...nd-below-spine . And you are sure you hit it?
It was ground level. I waited about 15 minutes. That's what I always do.