The rear texture does not show a gathering point. Fabric has to be gathered in order to drape, and that is usually at the shoulder. There really is no reason for the back of this dress to be draped. It would not naturally fall that way, unless it was gathered at the top of the shoulder.
If you look at a real dress, the pleated front is purposely gathered at certain points to create a pleated/draped effect. Then the front piece is attached to a flat back piece. I advise that you make the back texture flat, and make a seam at the sides.
That sounds right to me, you could have ruching along the back of a dress but you will notice that instead of it being pulled along the zipper, at the top it is just cut with no indication of how it is sewn in. As well, you will notice that the pleating and draping on the back is very organized, symmetrical and all of the shadowing is the exact same shade, this is what is contributing to the "ribbed" look. At the front you can see that the shadows are uneven (lightness, darkness, and thickness of shadows is varied) and the fabric drapes in a much less organized fashion (although still all going in the same direction). This is what is giving the impression of a much more layered, soft, correct fabric look.
On the back the shadows are all the exact same width and darkness, and there appears to be far too many of them (probably due to their similarity). The evenness and amount of the shadows is really adding to a very drawn, flat look as well. My recommendation is to add both darker and lighter shadowed elements and to tone down the amount you are putting on the dress. As well, you should really think about adding stitching and the impression of sewn elements (say along the top of the dress and the shoulders) as this will add to the "reality" (well, Sims reality anyway) of the dress. Take a look at some pictures of dresses like the one you are making in order to get an idea of the kind of stiching used (or at least how the fabric is fitted together as seams may not be obvious), as well as how shadows and highlights react with that kind of draping.
PMBD
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