No, I am sorry I don't know how to adjust the tension, since those horizontal drop-in bobbins aren't exsactly in a case. I think trying another bobbin, or rewinding the one you have is an excellent idea. Those bobbins can be touchy, and you may have one that is warped or worn in some way.
When you have the presser foot lowered and simply pull on the bobbin thread after it is brought up through the needle hole, can you feel tension? If not, do you feel more tension if you put pressure with your finger on the snap down lever that holds the bobbin in place?
I don't have that machine any more, so I am sorry that I can't help with more information.
Dayle
Dayle