The best way I have found to make sure of your seam allowances - and I do believe this is the culprit in most cases - is to cut 3 strips 1 1/2" wide. Sew the strips together long ways. At the ironing board - set the seams and then press the seam allowances out. Measure the center strip. Is it 1" wide??? Too narrow? Your seam is too big. Too wide? Your seam is too small. Once you get it RIGHT - put masking tape on your machine in the right place! It takes only a little bit of time and saves so much frustration.