The machine is skipping stitches or the stitches are looped.
Your sewing needle might be bent or dull. Carefully insert a new needle. The flat side of the needle has to face the back and the needle has to be inserted as far as possible into the designated shaft.
Then rethread the needle.
Please also check the combination of needle size/thread size/fabric:
we recommend using a thin needle and fine thread when sewing fine fabrics.
we recommend using a thick needle and thick thread when sewing thick fabrics.
The needle may have sewn on the needle plate cover or the presser foot, causing a misalignment. You should replace the needle in this case as well.
Last update on 19.05.2020 by Robert Lahmer.