It sounds like the piece of blackout fabric may not be 'squared' on all four sides. I made 21 panels for our sunroom (just used sheets for the lining). I was sailing along like a good little soldier and laid one out on a piece of lining that had gotten off square. I laid the top fabric on the cutting board or floor, smoothed the wrinkles out of the lining, pinned it down and stitched the seams. It is fine but still wavy in some places. Good luck.