It is skipping stitches.
Use a brand new needle that is a suitable type for the fabric and thread you are sewing. If sewing knit, use a ball-point needle. If sewing woven fabrics, use a sharp needle (ie universal,
Use good quality sewing thread--no old or bargain bin variety.
Be sure to install the needle facing the correct direction. On most machines, the flat side of the needle faces the back of the machine, however, if the bobbin loads on the left side (not from the front), the flat side should face the throat of the machine.
There could also be a burr somewhere that is catching the thread and causing the skipped stitch.
What to Do If Your Sewing Machine Is Skipping StitchesBasic Maintenance Tension Skipped Stitcheshttps://fashion-incubator.com/10_reasons_for_skipped_stitchesAll About Needles SCHMETZ Needles
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