What kind of fabric are you sewing? If it's thin, you may need to go to a higher stitch count - 12-15 per inch is usually good for quilting cottons, but for fine muslin or batiste, I would go with a finer thread and a higher stitch count - 18-20 stitches per inch. I'm not sure how that relates to the digital stitch count on your machine.
Does this happen with all bobbin threads on all the fabrics that you work on?
Unless you are intentionally working with a low stitch count and decreased tension, (so that you can gather the seam, for instance) a bobbin thread should not pull out this easily. At 12-15 stitches per inch, you should have to use the seam ripper.