The tension could be too tight or too loose. Set the tension to the basic thread tension setting or adjust the tension manually. The combination of the needle size, thread size and fabric is incorrect. Be sure to use the correct size needle and thread for the type of fabric that you are sewing.
What width should my sewing machine be on?
A straight stitch has no width so it makes sense to set the dial at “0”. However, each machine varies so you will want to set the Stitch Width dial at the position where the needle is centered over the stitch plate. This will give you the most accurate seam allowance.