your thread tension is too tight. Adjust your tension to a lower number. If the top thread shows on the bottom, the thread tension is too loose. Set your tension to a higher number. Adjust the tension until the top and bobbin threads interlock between...
If the bobbin is not threaded properly then the bunching is likely to occur. For this, take out the bobbin case and rethread it. This can also happen if the bobbin thread is too loose. In that case, adjust the bobbin properly to get the right tension. Uneven stitches: Stitches become ...