Before you take your machine head in, try this. Adjust the bobbin tension so that when you lay the bobbin in your hand (inside the bobbin case) and lift up on the thread, the bobbin case stands up on end but doesn't leave your hand. Next, loosen it a bit more so that when you are holding the thread and give your wrist a slight downward shake (like dropping a yoyo), the bobbin unwinds about 4 inches or so.
Next, loosen the thread tension on your machine head nearly all the way. Start sewing, and gradually tighten the upper tension until you can just BARELY see the bobbin thread peeking through.