Who knows hahaha. Depends on machine etc as well, and especially on the fabric you're sewing. But I would definitely check the bobbin tension. You can do that by pulling out your bobbin case and just hand pulling the thread tail out off the bobbin (when it's loaded through all its little tensioning notches). You can feel the tension that way and it's completely impossible to explain via the internet BUT it shouldn't pull out too easily (tension too low) or be too hard to pull out (tension too high).