Steven, exact same thing happened to me on my first bear of the trip....cept mine penetrated through with 9" of shaft out the other side. Lots of blood the first 20 yards...then 400 yards following a trail that was a drop here and there at best. Apparently, after autopsies on other bears, Doc said the loose coat and the angle of the bear caused the shot to possibly skip off the shoulder blade...loooking good from the angle it was originally, but nowhere near far enough down in the body for proper shot placement. 2nd bear however was shot while walking and went about 10 yards before dying.
Richard, My setup was 64@27 Morrison recurve, Pearson Deadhead, 580gr total Arrow weight, 185fps...4" of dirt on the other side (on the "good" shot), traveled maybe 5-7 yards. I have a theory (and it's worth about $.02 since I've only killed one bear), but I think bears die very easily when compared to deer or other animals that are typically "prey". I don't think a bear really knows how to instinctively handle something killing it.
The main thing is, shoot low in the cavity...it's a surprisingly small area. Here's some autopsy pics Ray took of my bear while Tippit and I worked on it: