Much frustration ensued. I knew it was something to do with how the bobbin was spinning, or not spinning, and I kept cleaning non-existent metal fragments out, removing, and replacing the bobbin. Sometimes, the spring would pop out again, I'd put it back, and the problem would not go away.
It all ended when I finally popped the spring into the correct place by accident, as I was cleaning. Thank goodness, I was paying attention and noticed that my finger nudged it 'just so' and it popped into place. I then removed and replaced it a couple times more, to learn exactly what it was that I'd done. It must be lined up correctly, pointing in the right direction, and it doesn't take a lot of pressure to put it in place. It really just pops right in if you know the trick. Tension problem solved!
FWIW, checking how the bobbin is threaded in an 830 is, how shall I say it? Annoying. That's it, I'll call it annoying. After lots and lots of practice, several you-tube videos, one-on-one training from a Bernina tech, and a minor hissy fit I got it down to a mirror in one hand and the thread in the other. Why Bernina didn't attach a mirror to the inside of the door, I'll never know.
The arrow is pointing to the white thread from the bobbin. It's coming out from under the spring that applies the right tension that is to the right of the thread. Even getting down on the floor under the machine, there is no view of the bobbin. Trust me on that, OK? If you have exceptional hearing and/or sensitive fingertips, you can hear and/or feel the thread pop into place. Then the mirror is not needed.