I'm working on a new pattern I purchased last weekend. It consists of three 1 1/2 " strips sewn together and then cut on a 45 degree angle to make eight 3 1/2" long strips....ultimately making a star. It isn't difficult, but time-consuming. I finished out 3 full blocks today and each one is bulky in the center....almost bowing out in the center. The background fabric is cut on a diagonal, but I've been careful to NOT tug on it. What am I doing wrong????