There are a few reasons your thread might be looping underneath. The most common reason is that the needle isn't properly inserted into the fabric. If the needle isn't inserted far enough, it can catch on the fabric and cause the thread to loop around. Another common reason for this problem is that the upper tension disk is too loose. The tension disk is located at the top of the sewing machine next to the spool of thread. If this disk is too loose, it can cause the thread to loop around as well. To fix this, simply turn the tension knob until the disk is tighter. Finally, make sure that your presser foot is down when you start sewing. If it's not, the fabric can move around and cause the thread to loop. If you're still having trouble, take your sewing machine to a professional for a tune-up. A loose tension disk or other small problem can easily be fixed by someone who knows what they're doing.