PaperPrincess is correct it might be as simple as the spring or even lint in the bobbin area . I was recently having the same issues with my most recent machine which has a drop in bobbin (all my others have a front loading bobbin). The entire bobbin assembly popped right out when the stitch plate was removed - who knew that that much lint could accumulate in such a small spot? Once I vacuumed it out it has sewn perfectly ever since. Check your manual to see how to clean the area.