Hello Marie,
Make sure you do not have a small piece of lint stuck between the tension discs, top tension about 4 or 5, make sure you have the bobbin in the correct way round, you should be able to pull the bobbin thread through with slight resistence, if it feels tight slacken the screw on the bobbin case about a quarter of a turn anti clockwise and test.
drivewithken
May 2010