Hi Vivien.
I'm sorry you're having such a hard time. I agree with the person who said to go through your operator's manual; make sure you are doing everything correctly. All my machines (four of them) require that the bobbin be turning clockwise when you pull the thread, so it's not interchangeable as to how the bobbin is inserted in the case. I'm kind of thinking your machine needs service for the tension, but it's hard to tell without seeing it. Also, try using a high quality thread like So Fine, from superiorthreads.com, in the top and bobbin. Invisible thread can be difficult to use in some cases.
Another great resource, besides Leah Day's site, is thequiltshow.com. This is Alex Anderson and Ricky Tims' site and it has tons of info and fun for quilters of all skill levels. You'll have to register (I'm not sure if you'll have to pay or can use the free registration). Then go to the "Classrooms" tab at the top and watch:
Free Motion Quilting
By Patsy Thompson
A Super Series on the basics of Free Motion Quilting
Good luck! Vicki