I am wondering if you have the thread hanging up somewhere... I know it's a pain, but I want you to unthread the machine completely...than take a thread and run it back and forth thru the tension a few times...you might have a small thread caught up in there. I also use both of the thread guide loops at the top of the machine, not just the one.
Have you recently changed brand of thread? Maybe the machine doesn't like this one?
Hope one of these helps.