I think your problem is with the bottom tension. If you look at the bobbin holder, you will see a very very tiny screw just below the slot where the bobbin thread comes out of the holder. You can tighten the bobbin tension by turning that tiny screw to the right. But be very very careful. Start with just 1/8-1/4 of a turn. See if that makes a difference. BTW I use my DH's computer screwdriver set to do this. They seem to be the only ones small enough to fit.