Leslie,
You might want to revisit
Baumgaertner's value-massing "lecture". This is one of the most important issues in translating life to two dimensions. Your darks are the same value in the lit area of the cast as they are in the shadowed area of the cast, which flattens the form.
Many others on the forum give excellent advice on value-massing; try your forum search feature with the terms "values," "value-massing," and "single light source".