Not taking control of the AF point
Don’t leave it up to the camera to decide where to focus. It won’t know which feature you want to be sharp in the picture, and if there’s something in front of the main subject, or the background is detailed, or there’s not a great deal of contrast between the main subject and the rest of the picture, then your Nikon may focus on these and not the subject.
You’ll get more consistent results if you tell your Nikon which part of the frame you want to focus on. For absolute precision, choose a single AF point. The centre spot is the most sensitive, although not best-placed for the most dynamic compositions. For an off-centre subject, you’ll need to use the ‘focus and recompose’ method: point the central AF point on the subject, half-press the shutter release to lock the focus, and then recompose the shot.
Alternatively, use an off-centre AF point that corresponds with the positioning of the subject in the frame. This is the best option if you’re taking pictures at close quarters; if you opt for the focus-and-recompose method instead, the shift in camera position can mean that the point you locked focus on is now at a different distance relative to the position of the sensor, and may actually be blurred.