Most likely, it's because you have on-camera flash that is being directed at one area. To prevent that shadow, you need back light. For a quick and simple (though obviously not perfect) backlight, have the person stand with a lamp turned on behind him, above his head so that the light hits the hair or top of head. However, make sure the light itself is not in the photo. What you're doing is helping provide a more even light, so the subject is lighted from both front and back.