You are right, the sketch renderer doesn't do transparencies (it just sketches in the transpareny pattern), but I figured out the problem was the "color" setting for the figure. It was moved all the way to the right which for some reason made the figure not render in sketch mode. When moved over to the left and put on a very low setting, the figure sketched along with the background. It seems to me though, even if the slider is all the way to the right, shouldn't it just add a lot of color to the sketch? Oh well, I will continue to experiment.