My limited experiences with hog are about like Riches. Read-- they die hard. I watched my dad hit a sow of about 200+ lbs behind the shoulder with a 165g Game King .30-06 at about 150 yards, and she only jumped and ran into the deepest, thickest, most tangled creek-bottem undergrowth you've ever seen. I went ahead and waited for her and her shotes to come down the creek. Only the shoats came out, so I eventually tagged an 80 lb-er for myself (tender and sweet; the best meat, I'm convinced!). Dad and our friend Doug never did find that big sow-- she made it too far. She was hit HARD, folks.
I've talked to many a person who's confirmed this to be very common. I know a guy who says his hog hunting got far more effective when he changed caliber to .22-250, because he then only tags them behind the ear. This, by the way, seems to be the best way to go, if you can make the shot; put your bullet just behind the ear, and they go straight down. Dad took a 250 lb hog 2 years ago with a .257 Rbts 100 grainer by doing just that. For a shote of less than 100 lbs, I found no trouble with a .30-30 shoulder shot.
We all know about the gristle plate, but the book is this-- Hog are tough, just generally. Heavy bones, low center of balance, hard, bristl-ey coat, and a "Never Give Up!" attitude means you better pack a lunch.
BTW, if it helps, here in TX, Parks and Wildlife puts out pamphlets on their extermination; they're considered quite the major pest to ranchers and farmers, and were not originally indiginous.
Sorry for your loss, but it sounds like you did what you could, in fair chase. What bullet, by the way? I hear great things about the Nosler Partition Gold 300 grainer.
Will you, too, be one who stands in the gap?