Hi, most likely your bobbin sensor is being blocked by something. Common causes are a dirty sensor. Q-tip on the eye of the sensor inside the bobbin door. Or there is fuzz blocking it. Next take out the bobbincase and the bobbin. Look in the bobbincase and you will see a round spring. This spring acts like a brake for the bobbin. Sometimes these springs will get moved and out of position blocking the little space that the bobbin sensor reads through. Reposition the spring so you can see through it and the bobbincase.
That should do it.
Good luck.
Rick Engel
Bernina tech

Your bobbin may be in backwards. match with photo in manual.

I have the same problem with my Bernina Artista 640. It sews 15 stitches and stops. It's not because of not enough thread in the bobbin.

Low bobbin indicator - you need to change to a fuller bobbin.