a. Please check if you are in Manual Focus Mode.
b. Please make sure you do not move when pressing the shutter button also the shutter speed is not too low.
c. Please check if the distance between the subject and the lens is smaller than the lens' minimum focus distance.
d. Please check if you have selected the proper focus point which is focusing at the subject.
e. Please check if the AF illuminator is obstructed.
f. Please check if there is something in the way while you shoot, such as a screen window or fence.
