There are several things to look at when facing issues with the auto-focus of the camera.

First of all, please check if the camera actually is set to auto-focus. Next, check if the person or object you're trying to film is actually in the center of the image. This is the area the camera looks at to find its focal point. Anything to the left or right of the image center is disregarded. 

If you find yourself in a situation where the person or object can not be in the center of the screen, you can consider using a manual focus. 

Manual focus, in general, is preferred in controlled situations where the distance between the camera and the person or object barely varies.