No, dabowhunter, the scope may sit an inch or more above the barrel, so the slug starts out low, and then rises up to the crosshairs at the distance the gun is sighted in for. Of course it doesn't really rise, it drops immediately, but the barrel is tilted upwards, shooting the slug up towards the cosshairs. Of course your scope would have to be mounted 6 inches above the barrel for the theory abovee to make sense, or be aiming at the bottom one inch of the deer's chest.
Here's my thoughts on this one.
A frontal shot at 30 yards with a shotgun is a piece of cake, I'd take it every time. Nothing wrong with that shot.
The idea that this deer is just "very strong" is ridiculous, no deer is "strong" enough to absorb a shotgun slug through the chest and live overnight, and then get up and run away. Just silly.
Either you hit brush, or just made a bad shot, but you hit the gut, or somewhere else outside the chest. Probably never know.
It happens to everybody, it sounds like you gave it an honest effort for a few days and looked pretty good. I wouldn't beat yourself up over it anymore, the only thing you'll find now is a carcass, the meat is certainly bad, you've lost the deer. If your curious what kind of rack you lost, by all means look some more, but maybe it's better you don't find it, ya know? Every time you see it you'll be reminded of the nice buck you lost. Anything you owe the animal is well past, the deer doesn't care at this point if you cut the rack off, or if the mice chew on it over the winter.