Hello Hester,
First of all just adjust the pressure on the foot so when there is a cotton fabric under the foot you can't pull the fabric through with the foot down, normally if you have a thudding sound when sewing either the end has gone off the needle leaving a square end or the machine cannot take very thick material, I can't think what that model looks like, on fine fabrics a size 11 needle is needed and special needles on stretch fabrics, if you sew on very thick fabrics you may need to turn the top tension down to about 3 or 4, always use a mercerised thread which is perfect all the way through.
Ken
ken
December 2010
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