I'm attaching a paint-over to demonstrate what I mean. Look carefully at the shadows where the folds are -- they follow the contours of the folds. Anywhere that you have shadows, keep in mind where the edges are in space -- further away from where the shadows are or closer? Is the surface where the shadows are flat, or bumpy? How would the shadows fall on a bumpy surface as opposed to a flat surface? I hope the picture explains it because I'm not quite sure how else to say it, but the shadows simply need to be less even and straight. An even better tip is take a piece of cloth and drape it the same way you have it drawn -- then look at where the shadows fall. ;-)