Sometimes fabric can slip a bit during the sewing no matter how carefully the pieces are measured. I've tried the following tips with success. 1) nestle the seams if they can be pressed to one side 2) if the seams must be pressed open, use a little Elmer's Washable Glue on the seam and line up the seams. Let dry or press with a warm iron to speed up drying...there is a nice tutorial by Sharon Schamber or 3) use u-shaped pins by Clover. It is like having a pin on either side of the seam.