I think it's the bullet's fault. I haven't heard much good about those 64 grn bullets. And 223 is a bit light for deer, it may have been a really tough deer. I always go for neck shots with smaller calibers to avoid your problem, but obviously you didn't do it wrong, it just didn't work out. I bet if it had been a 75 yard shot you'd have venison right now. But some deer are just really tenacious.
Once shot a big blacktail buck with my 44 mag. First shot at about 70 yards, and sneaking up on him put another one in him at 20 yards. He jumped and ran for 100 yards or so and I was so excited by his rack that I followed. Found him standing against a tree with two visible holes exactly where they should be behind the shoulder. So the next one went in slightly rear quartering. He jumped, huffed, and wandered away. I was standing there amazed the this deer had a perfect 3 shot group through the vitals with a 44 and good expanding ammo. So I waited about two min and followed. Again found him standing and not breathing(cold afternoon and any breathe was visible). Enough was enough, I double tapped him at less than 20 yards, there was a perfect 5 shot 4" group looking like a target on his side, and he ran off. I sat down for the next 20 min as the sun went down, went to my truck and grabbed a flashlight, then went looking. After those last two shots there wasn't a drop of blood anywhere. I looked 3 hrs that night, and 3 of us looked all the next day. Some deer aren't meant to be had.