As Brian suggested, use AF-S, then select your focus point. Do that ether with the multi selector in the back, disengage the shutter release button from focus, select the center focus point, lock it with the lock at the multi-selector, focus with the back button (AE-L/AF-L set to focus), recompose and shoot.
Those out points are not as sensitive as the center ones.
Also, your scenes are so different, head shots vs 3./4 or full length. If you are hand-holding, slight camera movement could be impacting your results.
Also, what picture control are you using? Portrait is not as sharp out of the box as Standard.
This bears more investigation, but you should look to do a good, controlled test to see if something is wrong with the camera or the lens.