I have more experience than I wish with poor shots. Some of that is due to having hunted deer with bows for more than 50 years (guns too). I've found some and failed to find others. Something that I have learned is that a wounded deer often does what we learn to expect such as go to water or bed in a weed "island" in open land BUT what I've come to remember is that a very hurt deer- one dying has confusion in its head sometimes and might do what we would not expect. When tracking a shot deer- especially when you aren't positive just how it was hit expect anything. I recall a gut shot doe that I knew I had killed. I remember tracking it to a bed and assumed that I had pushed the deer out of it. I shined my light all around as I stood next to a stove wood pile covered with an old green and white tarp that the landowner used to keep the wood dry till he came back. No blood trail- no sign of a deer. Next day I looked further in the opposite direction from where I had come the night before... nothing. A couple of days later I saw the vultures and went back in just to understand what had happened. The doe was right next to that tarp covered wood pile that I had stood next to the night that I tracked the deer. I had been 3 feet from recovery but remember thinking she wouldn't lay next to an old tarp! I had not pushed the deer from the bed... she had started feeling very sick with death coming and gotten up, staggered over behind the woodpile and lay down and died.
A little patience before the shot goes a long way. There are times when you are only going to get one chance and it's now or never, but most of the time, just being patient will lend a better opportunity.
Straight down shots are not ideal. It is very hard to get both lungs shooting straight down. Often if we let the deer turn to leave, then stop them, we can get a shot a little farther out, but it takes patience, and an experiential knowledge of how to read a deer's behavior and what he's going to do next. You can't teach that, you have to learn it by watching a lot of deer.