This is a pretty good likeness. But I think you're using the wrong process. I can tell by looking at it that you started with lines and then started shading. I don't recommend doing stuff like this that way. I find it better to start by laying down the base tone / basic shadows / basic highlights first, in large blocks. Then slowly refining it into the final details. With the bigger stuff it's better to start with blocks of color. It will also look more convincing later.

Quick example of what I mean ( yes I know it doesn't look like Robin Williams yet ):





It also helps to know the structure of the human face inside and out, because photos can be misleading and following that you can end up with situations like you have on the nose, where the placement of the shadows form a really strange shape. I guarantee his nose isn't shaped like that but following the vague detail on the photos, because of poor lighting on certain areas, you might be mislead into thinking so. So if you're going to do this often study that facial anatomy.