Sorry, I have no idea. Sometimes when I'm sewing something when I first start sewing I get those clumps on the underside from the bobbin thread too but it seems to fix and sew normally after a two seconds or so. ( not sure if yours does or not) I'm not sure why it did or anything either. I just thought it could have been the material I was using at that time since it doesn't happen all the time.


Again I'm sorry I can't help. I'd also love to know.
xox