Do you perform shooting in an environment where focus may not be adjusted properly in auto focus mode? In some environment, auto focus function cannot adjust the focus correctly. In such cases, adjust the focus in manual focus mode.
Under following conditions, focus may not be adjusted properly:
Shooting subjects that is in long and short distances at the same time
Shooting a subject through a dirty glass
Shooting in a dark place
With a glittering object nearby
Shooting a fast-moving subject
Shooting a low contrast subject