I agree with what Steven said, squint at your references and then squint at your portrait. The difference is marked. What really drove the point home in my own work was simple value studies in charcoal. You don't even have to tackle anything complex in these studies - simple still lifes will do.
One other point: your uniform detailing of the hair flattens the form - try first to render the structure of the head, the planes of the head, which show as masses of light/non-light in the hair. If you get these simple structures right, the detail follows.
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