My first thought when I read your problem was that you might have something in the bobbin case..i.e a dust bunny or a little bit of thread. To check this, take the bobbin out and run your sewing machine brush around the bobbin case.
Otherwise...
When that problem happens to me, I redo everything. I re-thread the top; re-thread the bobbin in the bobbin case, change the needle. Make sure that the thread coming from the top goes into each and every guide it is suppose to go into.
Make sure that the bobbin thread is entering the bobbin case in the right way... example, I had put my bobbin into the bobbin case with the thread unwinding clockwise and with machine I was using, the thread is suppose to unwind counterclockwise.
Good luck.