When does Autofocusing of your camera fail ?
The following subjects are difficult to focus and many a times, the cameras autofocusing, however advanced it may be, may not succeed in locking the focus.
Very Low contrast subjects - Plain Blue sky, Solid coloured walls.
Subject is in very low light
Extremely backlit or reflective Subjects - car with highly reflective body, mirror
Near and Far subjects covered by an AF point together - Animal in a zoo cage
Repetitive patterns - Skyscraper windows, computer keyboards.
This can take care of by focusing along where contrast is, but at the same distance as the subject. Lock the focus and then recompose to release the shutter.
You may opt to set the focus on manual and focus manually by rotating the lens dial at the desired distance.