This happened with my crescendo. Tried a old bobbin case and it fixed it. Ordered a new one and set the one that didn't work on the side of the machine. After a few days I noticed that the vibration had dislodged a VERY small piece of thread from the bobbin case. Now it works perfectly and I have an extra case! May not be the same thing with yours but check very carefully for dust or thread bits stuck in the machine.