hi there,
well, common problem when you didn't acquired right set up of tension and clean feed dog,bobbins,shuttle race.on figure right bobbins winding.
set up counter clokwise thread rotation, set dial tension on #3 to #4, dial stich to # 8 to #12
only on cotton fabrics,others fabrics such as silk..etc,gradually adjust tension fine the smoothest adjustment,
on fig,thread looping top side threading
follow threading so you can have fine sewing.also clean under bobbins from chips fabric and dust.have a nice day,don't forget to rate my fix,ythanks a lot