When seams gather or pucker, the fabric does not appear as smooth as it should be, which affects the appeal of the finished product. Friction between fabric and machine components, wrong combination of thread/fabric/needle, wrong stitch length or tension could cause the problem.
Friction between layers of materials
When sewing, the feed dog grips the fabric to help the material pass through the machine. Friction builds between different layers of fabric, between the lower layer of fabric and the feed dog, or between the upper layer of fabric and the presser foot. This makes the speed of passing through the fabric and that of the sewing needle different, creating gathered/puckered seams. Another situation is that the fabric layers move at different speed and causes great friction. You need to re-sew the affected parts, or use a suitable presser foot to lower friction.
Friction between layers of fabric built up during sewing may cause seam gathering/puckering.
Lateral displacement of weave threads caused by thick threads/needles
When the thread tension is set high, the needle pushes the weave threads aside during sewing (known as lateral displacement).This also happens if the needle is too thick for the fabric. If the weave thread position remains the same after the thread is pulled away, the weave thread position is changed due to the thermoplastic property of synthetic fibers. You may need to consider using thinner threads, finer needles, and a lower tension to avoid puckering, especially if sewing densely woven/knitted fabrics.
Seam puckering/gathering caused by lateral displacement of weave threads usually happens when densely woven fabric and thick needles are used when sewing.
Different material of fabric and thread
The degree of stretching is different among various types of fabric and threads, which affects the fabric to be sewn. If the thread stretches a lot, it will relax and gather the fabric after sewing. You can fix the situation by using threads made of similar material to the fabric to narrow the stretching difference.
Opt for threads made of similar materials of the fabric to be sewn.
The thread is not of low shrinkage nature
Some threads may shrink after washing or certain post-sewing treatment. This makes the fabrics gather after the sewing is completed for a certain period of time. You can use thread of low shrinkage to avoid the issue.
Threads of low shrinkage can avoid seam puckering.
The stitch length is adjusted wrong
A wrong stitch length means the thread is not sewn into the fabric firmly and/or with excessive/not enough space between one another stitch. When being pieced with other fabric parts, it puckers. You need to readjust the stitch length to re-sew the parts.
The stitch length adjustment is wrong if there is excessive or not enough space between one another stitches and the thread cannot be sewn into fabric firmly.
The thread tension is too tight
When the tension is set too high, the thread is stretched when sewing, then relaxed afterwards. Sewn with longer seams when stretched, the thread gathers the fabric, leading to a puckered seam. When re-sewing the part, readjust the thread tension.
A high tension may stretch the thread when sewing and cause seam puckering afterwards.