I had that problem when I wound my bobbin wrong. I suspect it might have been too loose. I got rid of one machine because screwing with the tension didn't fix it, bought a second machine and it worked wonderfully, until i tried to use one of my old threaded bobbins on it and had the same problem. Tried again following the instructions (yay for new machines coming with books) and it worked great!
Wife to one : Mama to 4 Gramma to 1 :