I wonder if your little squares moved on the snowball block? Did you mark the diagonal sewing line? Did you sew right on the line? I often leave the background square intact on blocks that add a triangle in the corner. I only trim the other half of the triangle. That way, when I press the triangle open, I can see if my block is off. When the triangle is larger than the background square I trim, when it is smaller I use the background square for alignment. Your quilt is pretty.