With the speed of today's bows, it can be very difficult to see arrow impact. I try to alleviate it by using bright knocks and fletching, but sometimes I still don't see my arrow fly. That being said, let's get down to your dilema with your buck.
I think if the deer was slightly quartering to you, you may have caught the near side lung, and went through liver upon exit and may have caught the gut or clipped the esophogus on the way out. If that happens to be the case, I would for sure look around the crick you mentioned in one of your posts, as he'll be looking for water. I would look by the crick anyway if it's the nearest water source to where he was shot. I would also look for vultures, and or crows that may lead you to your deer. GOOD LUCK, and I hope you find him.