How to fix the tension on the bottom part of the fabric with Jean material

On top of the fabric? The problem is either the upper tension is too tight and/or the bobbin tension is too loose.

Under the fabric (thread nesting)? The upper tension could be too loose and/or the bobbin tension is too tight. Also could mean the upper thread is not seated properly in the upper tension disk or it is threaded incorrectly. Remove the thread. RAISE the presser foot. Then rethread from the beginning--verify the thread pathway is correct.

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loosen the needle thread tension of a bit
sew on a scrap bit of material until you get the tensions right
correct tension is when the bottom thread and the top thread meet at the join of the two pieces being sewn
you will only need to sew 1" to see how it is going

To pull the thread through the material the thread tension for the top and bottom is too tight.

Does this machine have adjustable pressure settings for the presser foot ? If it does, reduce the pressure. The softness and bulk of fleece makes it tend to bunch, so less pressure is very helpful. A wide presser foot with a very flat bottom may also help.

Reduced presser foot pressure is helpful for most knits, very much so for stretchy fabrics. Fleeces are usually knits.

Using a stretch stitch also helps. If you don't have a stretch stitch, a very small zig zag is helpful to provide some give to the seam.

Bobbin tension depends on the thickness of the fabric that you're sewing on. A cotton fabric should have medium tension...but a heavy wool would need a looser tension. Best way to get the tension right is to take a few stitches, then look at the bottom side of the fabric. If the threads are loose, then tighten the tension one notch or number. Sew a few stitches, then test again. If the fabric is pulling up tight, then loosen the tension and check again. Top stitches and bottom stitches should both be the same.

Go back to the tension adjustments that were doing well with other fabrics.
You may need to go to a smaller size needle, and a finer thread. Sometimes I use a wooly nylon in my looper because it is very stretchy, but be careful when you pull it through as you are threading it because it will also break easily if you don't have your tensions full open as you are pulling it through.
Start out by buying a sheet or roll of Solvy. You can either cut it into strips to lay on top of the material as you sew or under it if you prefer. It should make the material lay flat as it sews. I have used tissue paper before under it to make it lay flat and keep it flat to make the looper thread stretch itself out as it sews.
If your looper thread tension was too tight on other fabrics too, that is a whole other problem.

try changing the needle--flat side to the back of the machine
the needle & thread & fabric have to be compatable
light weight fabric requires a different needle than medium to heavy weight fabric
if you do not have a manual check with bernina customer service about one
or
sewing partsonline.com
try a top tension of 3 or 4
clean under the needle plate & in the bobbin area for loose thread or lint

Not sure if I can help, but it may have to do with the heavy thread as well as the fabric and needle combination. If the fabric is thick or tightly woven, be sure to use a brand new sharp needle. (If the fabric is tightly woven (like Batik), a thicker thread may present a problem.) In your case, if using a heavier thread, perhaps a top-stitch needle would be beneficial since it has a bigger scarf to protect the thread. With the heavier thread in the top, the top tension will need to loosened. BUT, if you loaded the heavier thread in the bobbin, the bobbin tension will need to be adjusted to accommodate the thread thickness. However, remember that if you adjust the bobbin tension for this project, you will most likely need to readjust it for the next project using different materials.

FWIW, if the machine still works fine with regular fabric and thread, then you'll know that the issues revolve around the current project.

Understanding Thread Tension Threads

How to Achieve Ideal Sewing Machine Thread Tension Threads

Sewing Lesson 10 How to Fix Tension on Your Sewing Machine

The tensions must match ie. top tension =bottom tension. If the material is not sewn together tightly, that means the tensions are mismatched. Sewing machines will sew whenever the top and bottom tensions are equal (be it very tight or very loose).

We usually advise the customer to not touch the bobbin case tension. When that is not changed, all tension adjustments can be made from the top tension above the needle.

It appears that one of the two tensions (top or bottom) is much tighter then the other. I would reset the tensions to how they came, if you are unfamiliar with sewing machines this may take a little bit of experimenting until you get it back to a standard quality stitch.

The other option would be bring it to a sewing tech. A good mechanic should be able to adjust the machine and your tensions to your fabric (and recommend needle size for your thread/materials) in about an hour.

Are you using the correct type of needle? have a word with your needle supplier and explain the problem. Different threads and different thickness material need different top and bottom tension adjustments.
Can I suggest sew one thickness material at a time with tension adjustments for that thickness. Then change the tensions and do a batch of thicker material.