Since no one has described the blood (or even definitively confirmed it) its hard to say if any blood would be part of a blood trail, etc. Lots of small injuries can bleed a lot, like cuts on the head, ears, etc. But they are not critical injuries and probably wouldnt leave a big splattery trail that people or dogs could follow any distance.
Dogs are usually real good but not infallible either. It could also be that since the victim lived there, probably walked around her yard area enough, etc. that the dogs could not find one particular scent trail. Weather could play a part too.
One detail I had thought of ,that no one has that I know of, mentioned, is the possibility of natural trails around the family home, made by Holly/her family/ friends/etc. People dont just walk through the wild woods they have to make a trail first. And since her brother seemingly thought Holly walking into the woods was semi normal, there are probably trails opening up around the road in front of her house. Would any of these openings be large enough to park a car. It would be hidden from view, even if only 20-50 feet back in the woods. So Holly could be lead there, put into a car and then driven off. Now this does not account for the neighbor hearing screams but I dont know how close any of the neighbors are and perhaps she was driven to another location on/near/around the property either on a trail or on the main roads.