Skipped stitches is another common problem apart from the notorious thread bunching under the fabric. Whether the stitch has skipped one or multiple times, it weakens seam strength and the appeal of the finished product. Therefore if you have found any skipped stitches, you should remove tat thread and re-sew the affected part.
There could be various factors that lead to skipped stitches, but generally there are four major ones. You can check the machine if one of the following happens:
The bobbin/hook is damaged
When the bobbin/hook is damaged, it does not hold the lower thread well and is not sewn in sync with the upper thread, causing skipped stitches. Be sure to check if the bobbin or hook is in good condition, replace if it is damaged.
A broken bobbin/hook cannot hold the lower thread firmly.
The needle is not inserted correctly
Remove the needle by loosening the screw above the needle clamp, then re-insert it with the flat side of the needle facing the back. Place it without any space left in the shaft, and tighten the screw firmly. Before resuming sewing, test a few stitches.
When inserting the needle, the flat side of the needle should be facing the back. Do not let any space left in the shaft.
The needle is damaged
When the needle is damaged, it is not able to hold the thread well and stitch the fabric. If the needle is dull or damaged, replace it with a new one.
If the needle is blunt, bent or any other sign of damage, replace it with a new one.
The size of the needle/the thickness of the thread is wrong
If the size of needle is wrong, the thickness of the thread does not match the eye of the needle, which creates abnormal loops and thus skipped stitches. When the eye of the needle is too large for the thread (i.e. thin thread), there will be irregular/fallen loops; when the eye of the needle is too small for the thread (i.e. thick thread), there will be no/small loop. To avoid this, always choose the needle that works well with both the thread and fabric.
Skipped stitches occur when the size of needle eye and thickness of thread do no match.