My sewing machine is missing skipping stitches

Thread tension is too tight - Re set tensions. As a starter point set tension number to No.4
Needle is bent or blunt - Replace needle.
Needle is not the correct type or size for the fabric being sewn.
Thread take-up lever has not been threaded - Re thread and check thread goes through take-up slot.

Thread is of a lesser quality (too stretchy, twists and causes knots, too hairy and does not allow the machine tensions to control it properly) and needs to be changed for the correct sewing machine thread to suit the fabric being stitched.

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