I would have to agree that all of the good head shots I've made have taken the pig down...but I've seen them run from heart/lung shots (I know, that's normal) and from neck shots and they ran and fell over...not breathing...but still running while laying there on their side apparently dead.
As far as the big hog from Monday, I just missed the brain...simple as that. It was from the front and I've always seemed to have a harder time actually hitting the brain from the front. From the side, just put it in the ear. But from the front, I think it matters more the head position the thickness of the skull. Plus, I just miss occasionaly.