Bec is right. You've to half-press it to focus, then full-press to shoot.

This is not a bug, it's a feature *chuckle*. Seriously, this is an advance feature that you could find in other commercial grade consumer cameras. After your subject gains your focus, you can move your subject to somewhere else in the picture other than centre. Unlike other dummy phone camera, where you can only put your subject in the middle to have it focus correctly.