My feeling is that there's too much ambient light overall. The shadow areas seem to be gathering a lot of environment lighting that's lifting the low end values. This makes the whole image seem washed out and flat, so it won't really matter where you place your sun since the result will still be an overall lifted image. You could try post processing the image by lowering the gamma a bit but I think that your shadow and key areas are too close together to really make that trick work.
-Greg