Say, I love how much of a range of tones you've got from your semi-undressed model here, but the face, as long as there's a dark shadow on the right side of the face, this needs a light tone. Would you try adding a light on the left side of that?
During your next practice session, kindly set up a desk lamp to shine against some objects for a thorough observation and caricature of light and shadow.
Hope this is totally and completely practical. For more info, please rent or buy some copies of drawing tones. Keep it up till the end.