Perhaps the problem it is your bobbin tension. Provided you have a bobbin casing that is not a drop-in-type, try holding the casing thread by the tail and allow the bobbin casing with thread caught in the hooks, to hang downward. Give it a little shake as you would a yoyo. If it does not slide at all, then you need to turn little screw on the side of the bobbin approximately "15 seconds" (as in a clock) counterclockwise. Continue 15 seconds at a time until it slides slightly, a little at a time when you shake.
However, if it slides too easily, then you need to tighten the same screw (clockwise) 15 seconds at a time until it slides a little when you yoyo.
I know, I know, we are told not to mess with the bobbin tension, but there are times when it is necessary. This may be your problem.
Suz