Put a piecing guide on the side of your machine to make sure you are getting a true 1/4 inch seam allowance. If the seam is correct, check that the tension is good and not slightly gathering the seam line. Too many stitches per inch can cause curving on the seam line. I also press the stitching line flat before opening the pieces to press to one side. Make sure you are pressing your strips and not ironing because that can distort strip sets.