I am sewing a skirt with a lightweight woven fabric(a little silky and slippery). The side and back seams are puckered. I'm using Gutermann 100% polyester thread, a new regular needle size 11, 2.5 stitch and balanced tension I pressed the seams and the puckers are still there. I ripped out a seam and sewed it thinking that I didn't sew straight enough, it did not help my problem. Any advice on what I could be doing wrong.