If you are watching the arrow in flight, you are taking your concentration off the spot you are aiming at!

I do see the arrow hit, but not because I watch it in flight, when you are concentrating on the aiming spot, you will see the arrow enter that spot, assuming you hit it. However you have to realize what you are seeing, the arrow is passing through that spot so fast that all you may see is a small "blip" in the fur of the deer, that's the fletch/nock passing through!