A lot of good possibilities listed above. Here is another one that I don't recall seeing, but it's happened to me. With a quartering away shot it's possible you hit where you say you did. It's also possible that the RIB you heard crack was actually the arrow penetrating and hitting the offside shoulder or leg bone. In many cases when this happens the arrow actually bounces back. In such cases you may have gotten complete penetration without a pass through. Couple that with maybe some fat or tallow partially plugging the entrance hole and you're on your way to finding out about your true hunting skills.
Without an exit hole the lung cavity has to fill up enough to spew blood out the entrance hole. In the second or so that it takes a deer can take off and cover a lot of ground. Maybe 50 yards before the first sign of any blood. If running full tilt that can mean a very spotty blood trail; one that takes a lot patience to follow.