Sorry about your apparent loss. Sounds like a further grid search is your only option. Interestingly, I had a similar experience on a very similiar hit earlier. The only reason we found him was because we read not only the blood trail but the direction of the trail - the way the splattered blood pointed, the side of the limbs and brush that was painted, etc. Turned out the deer had backtracked and this was our only intial clue. We eventually did find some hoof prints "going the wrong way" to help prove direction along the same backtrack and the subsequent new trail. Maybe there will be some prints left with your buck. Good luck.