So you're saying you double lunged her? That does not sound like a marginal shot to me - that's a good shot. You are concerned that the bloodtrail on a double lung shot was light but that could be due to a variety of reasons. Did you hit her high in the lungs? If so the blood is going to stay in the cavity instead of spilling out. How sharp were the broadheads? It is possible that you didn't hit any of the major arteries in the lungs. These things happen regardless of broadhead type.