Check the thread path from the spool to make sure it hasn't wrapped around something. I usually sew with my top tension at the red line or up to the # 5 depending on the thread. Check to see if your thread is a little thicker then what you usually use. Is the spool in the correct position to flow freely? If all looks good on top, it is time to check the bobbin tension. Look at your thread sample and see if the top thread is being pulled to the bottom. Your bobbin tension might be a bit tight.
How long has it been since you put a drop of oil on the bobbin hook? I give mine a drop after about 5 bobbins. If all that fails, change thread, needle and clean your machine and try again.