This happens when the looper or hook fail to enter the thread loop during the stitch formation. Possible causes are Incorrect timing of the looper or hook Incorrect setting of the take up or pull off devices Needle vibration or staggering Wrong size needle hole plate Needle deflection by resistance of the material Loss of thread control due to the wrong size needle being used for the thread used in sewing. 　 Lock stitch skipping Chain stitch skipping