Do you cut the triangles first, or use 2 squares, draw a line down the center, then sew on either side of the line then cut? In either case, your seam is on the bias of the fabric. Sewing a square give a bit more stability to the bias edge. I'm thinking that you might be tugging on the bias, stretching it. a bit so it goes haywire. Using the stiletto is a good idea, but only use it to guide the fabric, not pull or push.