Try this procedure as follows:--
1. 1)Choose a piece of scrap fabric for practicing stitches. Sturdy cotton is the best to practice with, jersey or silk like fabrics will be difficult due to the elasticity of the material.
2. 2)Insert and tighten a needle that is appropriate for the fabric. Be sure that the needle is in straight and is not loose in the sewing machine.
3. 3)Put on a straight stitch presser foot. Pull up your feed dog so the fabric pushes through smoothly. Keep the presser foot lifted.
4. 4)Fill a bobbin with thread. Thread the needle. Insert the bobbin. Use the wheel on the side of the machine to manually push the needle down and up to pull up the bobbin thread.
5. 5)Set the machine to do a straight stitch. On older machines this is a stitch length of 3. Set the dial at the top right side of the machine to straight stitch. The machine's instructions show how to align your particular machine.
6. 6)Set the needle to the center of the presser foot. Place your scrap fabric underneath the presser foot. Release the presser foot.
7)Slowly push down on the peddle to start the sewing machine. Use fingers to guide the fabric through while keeping the fabric straight. Ensure that the fabric is at an even seam allowance by keeping it even with the ruler on the right side of the presser foot.
7. if following this procedure does not helps then its the problem quite with the needle, bobbin or the head it self.the alignment in the head is getting slipped so its not moving the thread and just gathering the thread.
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Hello. When gathering is a problem, set your machine to a shorter stitch.