long stitches or tight tension can cause puckers along a seam. shorten the stitch length and loosen the needle tension to see if this improves the seam. if the problem remains, pull gently on the seam in front of and behind the needle to support the stitching while sewing. to achieve a smooth join, iron the seam flat first then open and iron the stitches from the wrong side. finally,hover the iron over the surface of the seam lightly.