The technique you are using is one that works with traditional video cameras which is probably where you learned to do this. Still cameras and their lenses are different – they don’t always hold focus when you zoom back out….which I believe is the issue here.
So what I do is I use the camera’s LCD screen on back (Live View) and then use the magnifying glass type button to zoom in to about 10x digitally (on the subject’s eyes ideally. ) I focus and then turn the magnification back down to the normal setting.
The white box on the screen (for my Canon anyways) is the area the camera will magnify into as you digitally zoom in for focus however you can move that white box to wherever you want it which saves the day!
Steve Crow
Crow Digital Media
http://www.CrowDigitalMedia.com