another reason might be a dull needle. make sure that you're using a new needle.
if not try this (will require a little more work) saw this on another forum. might be helpful.
remove the bobbin case and bobbin., turn the machine so you can see the needle as it enters the area where you removed the bobbin case. (for clarity, this area is commonly called the hook and race area) as you turn the handwheel, you will see a curved or semi-circular piece turning towards the needle. This part, called the hook will have a point on it which should pass behind the needle approx. 1.5 to 3.0 mm above the eye of the needle. If this is not the case then either the needle-bar or the hook are out of timing. This applies to all sewing machines of any make or model. This can be a simple repair if the needle-bar is too high or too low, but more difficult if the hook is incorrectly timed.