I rendered the dress with the 0.5 shadow and when it was done, I noticed something that might have a baring on things. Look at the bottom of the gown, where the trim is. At that point, the light only has one layer of gown and pattern to get through- and there is a difference in the shadow. BUT, looking behind her, past both sides of the gown and her, we get the totally black shadow. I'm thinking there's too much pattern in the gown to get the difference in the shadows like I'm trying for.