Hello Caroline,
If you are absolutely sure that you have the bobbin in the bobbin case the correct way round, and if you are sure you have the needle in the correct way round, just make sure that you have the machine threaded correctly and the thread has gone through the tension discs, very common fault that one, make sure you have the top tension set properly usually between 4 or 5, if you have checked all of this the machine should sew perfectly, if it is not sewing perfectly let me know, e-mail me.
Ken
drivewithken@blueyonder.co.uk, August 2007