Did you do a post mortem and track the wound path? That would tell you a lot about what the head encountered and why you didn't get more than 10" of penetration. What condition was the head in when you recovered it? Still straight or was the tip rolled (the one thing I don't like about Steelforce heads is their weak tip).

Just something else... and I'm not faulting you at all, just making a suggestion... if the deer dropped on the spot and lingered for that long, I would have put another arrow into her chest to speed up her death. Something for you to consider if you ever find yourself in a similar situation.