Either you shot it high and went off with your arrow. Or you hit the shoulder and it could have run off with your arrow. Chances are you hit in high and the arrow got broke off in the bushes.
I've hit them high before and had a complete pass through, and found only little blood. A heavy blood trail is an indication your hit it right. What the heck, it's a hog .... Next time try to get them quartering away just a little bit and stick in just behind the shoulder and let the arrow come out the from. I've never seen an arrowbounce off an animal, even on bone.