It would help if you could supply more information.
- What auto focus settings were you using? - How did you determine the focus point? Visual of the image or where the camera actually focused? - Provide an image of the deer with the EXIF data available.
The depth of field at f8 should have put much of the deer shot in focus so that part is a little concerning, however, without seeing the image it's hard to make any comments of value.
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"