Like you, I have also had trouble with that inner material being flat. Ditto to what HandcraftsbyJen and Peckish wrote. By trial and error and I finally quit using so many pins and now I do what Peckish does. I sew just past that seam, stop, and straighten. Then maybe use one pin if I feel I need it. If I am really having trouble, I will even take the block out, get the material in place and then return it to the sewing machine to complete. Let us know how it works for you.