i find this happens when an accumulation of lint builds up in the bobbin.
Also lubricate your bobbin and make sure the thread is flowing consistently out of the bobbin. This happens to me occasionally when i use certain fabrics that have a lot of lint or dust that comes out of it. and cleaning and lubricating it seems to fix the issue.
Also lubricate your bobbin and make sure the thread is flowing consistently out of the bobbin. This happens to me occasionally when i use certain fabrics that have a lot of lint or dust that comes out of it. and cleaning and lubricating it seems to fix the issue.