Their are lots of possibllities, if you focus and recompose, if you lock focus by pressing the shutter halfway and then sway forward or backwards a small amount, if you are near minimum focus distance, most lenses will front focus, so its best to test it carefully with as many variables removed as possible.
Place it on a sturdy tripod, full aperture, ISO 100, and take several shots at about 25-50 * the focal length in use. Then go to the same focal length and distance used when taking the image and take 8 or 10 more.
Focus on a flat target so that you can be certain the camera selected the correct place to focus. The camera will often focus on the closest object, even if its not in the center of the point used.