the video Bebe pointed us toward is helpful if you have a machine with a "stick-in" bobbin case. we have drop-in bobbins. it is possible to adjust the tension on a drop-in case. it is also a huge pain in the you-know-what.
you didn't mention how long you've had the machine or how many "miles" you've "driven" it since purchase or last service.
if you have already tried switching to a new needle of a larger size, and/or heavier thread in the bobbin, it may be time to take it in for a tension tune-up. i finally did that with my simplicity a few months ago and it has made a world of difference. i hated to spend the money, and i still think it cost too much, but i filed it under "what-the-heck-i-waste-more-than-that-on-pizza-delivery-and-fabric-i-definitely-didn't-need".
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