I agree with PABowhntr - it could be any of the things you listed. If I had to guess - maybe the brush in the background. Maybe if you could find a tree with a limb in the right place - you could attach the camera high and almost facing down on the deer. That would eliminate the brush in the background. Just a thought.

I think I even saw a post where someone placed black electrical tape on the right and left edge of the sensor. That eliminated the sensor from picking up the motion and snapping a pix when the deer wasn't quite in the pix frame.