Shoot again anyway. At that point he's hit and wounded so Range is not a concern. Take a quess at how high to hold and let it fly.

why weren't you watching all along? If a deer drops immediately to a bow you should be worried. You've either spined them or they dropped down to hide. I've seen that happen personally. I shot at a deer and hit a limb that deflect the shot over his back. The buck dropped straight down. I guess he didn't know where the shot came from. He started belly crawling away. No BS I seen it with my own eyes.

heck, if I shoot an animal with a rifle and they drop in their tracks I cycle the bolt and keep them in my crosshairs for a couple minutes. I've helped track too many animals that "dropped in there tracks"the story is always the same: "He dropped so I thought he was done, Then all of a sudden he jumped up and took off."