I had a similar story. I've had my 12ga. USHD for 5-6 seasons now. I take mostly rested shots from my blind, but out to 110yds or so. I've only lost two deer that I've shot at with this gun. Both bucks. One was just a nick (found hair but no blood). This year's was even worse... probably 90yds, broadside, rested shot, aiming at the lungs.
I don't know what happened... I thought it was a good trigger pull... He jumped and ran 30yds. and stopped. I figured he'd drop over in a minute. After a few minutes he just trotted off. I figured I must have clean missed him, it was weird. I'd never missed a boiler room shot like that. Sighting in I was getting touching holes at 100yds... I checked and I found blood. My oldest son (8-1/2) and I tracked him for 3.5hrs through about 1,000yds of woods and the trail dried up and I lost it near the edge.
Very embarrassing... The best I could do is tell my son: "I try to be accurate and I try to bring them down in 1 shot. But when that doesn't happen, our responsibility as hunters is to give it our best effort to find the critter and put it down. Well, we gave it our best effort and this one got away. Let's hope that someone gets it or that the wound wasn't mortal..."
It's good to know that it happens to us all...