autofocus - Why does my DSLR focus on the background instead of my subject when taking shallow DoF photos?

I am an amateur photographer and have a Nikon D7000 and I want to get the blurry background effect (shallow DOF). I have a 35mm lens and I have set the aperture to be 1.8.

While taking a picture I put the small box on the person's face and then semi press the shutter button and after it focuses, I take the picture. However, when I see these pictures on my computer, I see that the background is sometimes in focus and the person's face is blurred.

I suspect there is a problem with the autofocus, any ideas how can I fix this? I can’t make out the difference in the viewfinder, so its difficult to tell how the picture will appear enlarged.