Looping under fabric and Irregular stitches
Stitches that have a small loop protruding on the top or bottom of the fabric often are caused by incorrect tension settings or a tiny burr on the hook. (The hook is located at the back of or under the bobbin case).
Another cause would be the bobbin case has been hit by the needle and is damaged. This is much more likely if the machine has the black plastic bobbin case fitted. This type of bobbin case can easily become damaged by the end of broken sewing machine needle or even if a pin has found its way into the bobbin case area.
First check the clear plastic spool/bobbin, in fact if you are having trouble I would throw it away and use a brand new one, not a cheap copy as they are not manufactured to the same quality as the originals. Speaking from experience cheap spool/bobbins will cause a machine not to sew well.
It is easy to check the bobbin case for damage as it can be lifted out and inspected after removing the needle plate first.
Run you finger nail along the edge of the plastic bobbin holder and feel for any kind of indentation. It is extremely important that this edge has no indents or grooves. The thread runs around the bobbin holder so it must be perfect. If you find damage then replace the bobbin case.
When replacing the bobbin case make sure it is positioned correctly onto the hook edge and any securing tab re-positioned before replacing the needle plate. Remember whilst you are in there it may be worth giving the area a clean out with a small lint brush, plus one drop of fine quality machine oil between the surfaces of the case and hook will work wonders to keep the machine quiet and running smoothly.