Quality bobbin thread is extremely overlooked. If you are buying giant cones of bobbin thread for a cheap price, you are causing more problems than you know. And pre-wound bobbins are convenient, but they are not made to give optimum performance in home embroidery machines as each machine is made to use with a specific size and weight of bobbin. Just because the pre-wound bobbin fits does not mean it is performing correctly in the bobbin case. Pre-wound bobbin threads are also usually cheaper, fuzzy, inconsistent thread remnant fibers as well.
Take the time to wind your own specific machine brand bobbins with a quality bobbin thread and you will have better embroidery stitch outs. You wouldn't use cheap embroidery thread in the needle, why would you choose to use cheap inferior thread in the bobbin?