I'm in the middle of sewing a dress and have just added the zipper (which I had originally taken off for no reason other than I didn't like the look of it, now I realize that I want it on again), it's decided it wants to pucker near the small of my back. Is this a problem with the fabric and that I need to re-sew the zipper, or is there something I can do to fix this without having to seam rip it? The material is polyester, and was originally a seventies prom dress that I have butchered. Any help is appreciated as I don't have much time left to finish it.

Can you rip out just that section and resew it? Iron before you resew. Right now you have too much fabric and too few stitches, so resewing should equalize that.