There are many reasons that your seam might not look right. When you are sewing on different fabrics, you need to adjust different things in your stitch including width, length and type of stitch, but one thing you might not think about is tension. Tension is how tight your top or bobbin thread is. You should always sew on a sample piece of fabric before you sew on your project. When you do, you may notice that the stitch looks loose or tight. What you should do next is adjust the top tension. That is usually the one with the dial. It is usually found near the top of the machine. (Your instruction manual should help you find where it is.) You don’t want to adjust the bottom tension as this can mess up your sewing machine.  After you adjust the tension, you should sew another sample to make sure the stitch looks right.