Help! I went to do a test piece and the bottom is very bulky - like there is a ton of thread at the back. I changed bobbin thread and the needle and that only makes a little difference. What else can I try?

I know that you have changed the bobbin but have you taken the cover plate off and the bobin case and given it a good clean as this happens to mine when a piece of cotton is caught somewhere in the mecanism. Hugs Joyce

ttt

Do you mean that the back of the work is looping? Is there tension on the upper thread when you put the presser foot down?