I am looking at your pieces, what they tell you to do, and the picture of the quilt.
If you look at the finished quilt. The middle pieces are put at the top of the left triangle causing the pieces to start angleing upwards, then do the same with the right. I wish I could draw a picture, but, look at the finished quilt closely and look how the center is placed on the top of the triangle, rather then where you have it