Yesterday I was having a lot of problems with my bobbin tension because I bought some new bobbins that don’t fit the same way the other’s fit (even though they’re both 15 >.>) so have a little interactive guide on how to troubleshoot tension problems.
I’ll go a step further and tell you that you can change the tension on the bobbin (this is only on some machines that have a bobbin case) by tightening or loosening the screw on the bobbin case. The “Screw driver” that your using is the one the machine came with, My Brother back at Uni actually has one that looks like a screw driver and not a plate. However the plate is better in my opinion.
Just make sure that if you’re making it looser you don’t make it so loose that the things comes apart. If it does sweep the area thoroughly grab some magnets and see if the magnets can pick it up.
The top image is from this site.