I don't care what you shoot, sometimes even a "perfect" shot can take a long time to kill a deer. I shot one with fixed blades through both lungs and he went about a kilometer and I wouldn't have found him if I didn't know the land so well. Just bad luck sometimes. If you shoot enough deer, that 1% will happen.

That said, it could be that your set up isn't powerful enough for the large cutting surface of some mechanical heads. Something to consider and do moreresearch about.