What kind of needle are you using and is it in properly? Is the bobbin in correctly so that the thread end is heading to your right when you pull on it before running through the little guide? Rethread your machine using the book for each step. Is there lint build up in your bobbin area?
Not sure what else it could be, so would suggest bringing it in and letting them determine and ask that they do everything with you watching so to help you in the future. Good luck and I feel your frustration. HUGS