I've been on one hunt where we lost a bear that we knew was solidly hit. Friend is a very good shot and had a solid hit on a big bear (watched through the spotter by another friend up above our basin). Shooter saw him do a summersault at the hit, and assumed he was down. We found blood, and grid searched the area looking for more, searched the entire mountain side looking for more than the 2 big liter size splashes we found of bright red blood. Followed tracks and scuff marks into thick brush, and never found the bear. Mistakes we made in hindsight: 1. Once the friend on the spotter saw the hit and saw the bear tumble, he assumed he has dead, and headed down off the ridge to help break it down. Didn't watch it until we recovered it. 2. We were in a big V shaped basin with thick brush and deep V cut ravines. I came down to help him recover the bear, didn't see the shot myself, as I had been working in on him from a different angle, and was out of sight. Friend was flustered, had found the blood, and saw the scuff marks from the tumble at the shot location, but couldn't find the bear. My mistake was assuming he had missed something being flustered and re-searched the area looking for more blood and sign, instead of getting to another high point and helping him look with my glass into areas we couldn't see from the ground. In hindsight, I wasted time re-covering ground, instead of expanding the search area faster. We might have recovered that bear if we hadn't made these two assumptions. These were all very experienced, skilled western hunters, with many animals and tracking jobs under their belt.
First elk I ever took with a bow, when I was very young, was a less than optimal hit, at the outside edge of my range. Penetration wasn't good, had hung up in the shoulder, I saw the arrow wagging as he ran into the brush. I let him be for a while then started tracking. Blood was spotty but had a good line of travel from scuff and stumble marks. Tracked him until dark, about 1/2 mile. Marked the spot, as the blood was getting thin, and went back to camp to get help. Came back in the morning with 2 friends at first light and we picked up the trail from my flagging. Blood petered out, so we started grid and spiral searching. About another 200-300 yards from where I ended my track the night before, we found the arrow sticking out of the ground, point up, in what looked like a 5-gallon bucket of blood. Kept grid searching and found the bull another 50 yards down the hill. Bull had laid down and pushed the arrow in and finished things. Point being, don't give up, even if you know the shot was less than good, mark every spot of blood, or other sign indicative of a wounded or hard running animal, and get help if you can't find it yourself.