Lots of things-how high up was he if in a tree stand? If arrow did not completly pass through but is sticking out the bottom it may be plugging hole for blood to come out bottom. If not a lung hit this could be reason for sparce blood also-sometime a high lung won't give you decent blood for several yds. If he's confident he made a good shot & you waited the hour go back in DAY Light. Lots more shows when you can SEE direction deer is traveling ect.. Keep us posted. If you still can't find the deer, do a grid search-you know, section at a time in all directions from last blood. Unless he died walking check all thickets & high grasses... Good Luck...