Today I went searching on-line for information on how to capture a likeness in a portrait. I came across a great page that answered just that question. It shows how a likeness is not achieved by the placement of the features or the light and shadow but rather by the form of the face.
This really helped a lot because previously I had been drawing in the features and then attempting to match the values. For today's painting rather than look at the light and shadows I looked at the form of the features. I related it to painting an apple - I don't look only at the light and shadow but rather the curve of the apple and the roundness of it. I wasn't working in the 3rd dimension like that with my portraits and it showed.