Your bobbin tension changes...
SETTING THE BOBBIN TENSION USING THE BOBBIN TENSION GAUGE
The Bobbin Tension Gauge is used to set the tension of the thread in the bobbin case.
Bobbin tension is measured in millinewton (mN). The recommended bobbin thread tension is 220 mN.
Bobbin tension should be checked if you change thread brands, weights, or type, including purchased pre-wound bobbins. Even different colours will tension slightly differently. Different wadding can also make a difference.
We check tension on each bobbin and sometimes part way through a bobbin.
We run a little piece of fabric along the excess wadding and backing fabrics required to hold the quilt onto the frame. We test the bobbin each time we change it and for each quilt.
See this photo
This video gives you more information about how to use the tension gauge. When pulling the thread through the gauge you need to ensure you pull the thread evenly and consistently.
If you are pulling evenly and you find the thread keeps jumping around in the gauge, the problem can sometimes be a little bit of fluff or lint or dust caught under the spring in the bobbin case.
Carefully clean this out with the cleaning brush or maybe carefully take the spring out to clean underneath it. Use the fine Bernina tweezers. Reinsert the spring. This will hopefully fix the problem.
Here is a link to the video showing you more of the process.