if you use a good quailty thread , i use a 100%cotton in top and bobbin . if you are ironing the seams with a to hot of iron the polly blend thread, the polly will melt away and you only have a small strand holding the fabric together and the seam will pull apart. Ask Me How I know . I do use a 2.0 stitch lingth and back tack on all outer seams . try the polly thread and a hot iron and see what happens . DottieBug