To re-adjust the tension on the bobbin, put the bobbin in the case as normally as you would to put back in the machine. Hold on to the thread tail and just by the thread you should be able to pick up the bobbin case unit without more thread pulling off the bobbin. Then gently bounce the bobbin unit. A little thread should spool off the bobbin then stop. If no more thread feeds off the bobbin the tension is too tight. This will help re-adjust the tension and get it quite close. It may need a little fine tuning after this which can often be done with the upper tension.