I am very skeptical of those who have always killed everything they every shot at, all one shot kills and all DRT. Sometimes funny things happen and you don't know or can't explain why. Is it possible to have bullet failure? Absolutely. Animals can respond differently to seemingly the same hit - was it breathing in when the bullet hit the lungs, lungs full/empty? One game warden I know in Europe, who has witnessed thousands of roe deer and red stag shot even figured that the blood pressure difference from the heartbeat at the moment of bullet impact might make a difference. I don't know, but there must be a reason why some animals collapse from a double lung shot while others run off.
I shot a sitka deer once from about 75 yards if memory serves me well. I was using a .270 with 130gr Ballistic Tips. My friend was watching in his binoculars. The 2 deer were on a little hump and right beside some very thick foliage. I shot and the deer just walked into the bush. No reaction, nothing. He said "I think you missed". There was no way I missed, rock solid, steady aim, not even a whisper of buck fever. No way I missed.
Well we (my friend, wife and me) looked and looked and could not find blood, hair or dead deer. Meanwhile, my dog had run off somewhere and wouldn't come back. She finally came back to my call and stood off - tongue hanging out and would not heel. My friend went to grab her collar and happened to notice the tiniest drop of blood. My dog turned back to the bush and we followed. We went about 75 yards through thick crap and the dog led us right to the dead deer, which was under a deadfall.
Perfect behind the shoulder shot, with exit that was much smaller than expected. Why was there virtually no blood, how could it go that far before dying and why was there no reaction to the shot? I had shot several before on that trip with the same rifle - as had my wife (her first deer ever) and most of the shots were bang-flops or a big jump and a step or two. Can't explain what happened on that odd one though. We never would have found it without the dog. The only thing I can thing of is that the bullet didn't expand properly - that just going from the look of the wound as I obviously didn't find the bullet.
So stuff happens - it isn't always black and white.