I think Suz is on the right track - one suggestion for those who can adjust their case's screw tension - Before you pull out the screwdriver, mark the original with a dot of white-out or a Sharpie. It sure makes life easier when you set it back for regular sewing.
I have a drop-in bobbin, too. Tension hasn't been an issue so far, so I would have to look it up in the manual. I find it helps to switch the machine to zig-zag when adjusting tension. The differences show up better.