When a bird's nest of thread appears either on top of or underneath your fabric, a possible problem is the upper thread tension. If loops appear on the underside of the fabric, the upper thread tension may be too loose. If knots appear on top of the fabric, the upper tension may be too tight.
1. Rethread. Remove the thread spool and bobbin; rethread your sewing machine with the presser foot up. Make sure the presser foot is down while sewing.
2. Replace Needle. A damaged or dull needle may be the problem. Change the needle and make sure it is the correct size and type for the fabric you are sewing.
3. Testing 1, 2, 3. Use threads of different colors in the needle and bobbin to stitch a test seam. If the seam pulls or bunches, it is easy to determine if the tension problem lies with the top or bottom thread. Adjust tensions and test again.