Reason No. 7 Your Photos Aren’t Sharp: Continuous AF not selected
In single AF mode the camera focuses the lens when the shutter release is pressed half-way and the focus stays at that point until you lift your finger off the shutter button and press again.
If you want to focus elsewhere you need to lift your finger and either recompose or select another AF point before depressing the shutter release again.
If you use single AF mode to photographing a moving object you’ll find that the camera focuses the lens on the subject at first, but that the subject moves out of focus. Consequently, if you are shooting continuously only the first image will be sharp.
As it name suggests, in continuous AF mode the camera continues to focus the lens for as long as the shutter release button is held down.
This means that provided you keep the active AF point over the subject (many cameras offer a tracking options to help with this) it will be in sharp focus in every shot (within reason).
Reason No. 8 Your Photos Aren’t Sharp: Continuous AF selected
Continuous AF mode is enormously useful when shooting moving objects and provided the subject is under the active AF point it can also be used to shoot stationary objects.
However, if you use the focus-and-recompose technique the camera will refocus the lens as soon as you recompose the image.
So the simple rule is, if you are going to focus and recompose, make sure the camera is set to single AF mode.