Are you shooting in conditions which focusing is difficult with the auto focus mode? Auto focus has trouble focusing under certain conditions. In this case, the manual focus mode can be used to focus the camera.
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