My bernina 820 keeps breaking the thread while quilting

Not sure if this will help, otherwise, I believe Bernina has a Support contact on their web site:

Bernina 770 14 Cleaning the Thread Catcher

the bobbin casing is not clipped in place properly, lift the lever and push in place then lower the release lever.
or try another bobbin casing, these are only 30p on Aliexpress with free shipping.

according to this, approx 38 lbs

Thread Owners of the Bernina 820 with Quilting Frame

going to have to learn to love it somehow

Be sure to ALWAYS RAISE the presser foot when threading the top thread--this releases the tension disk so the thread will seat properly.

Sewing Machine Problem 1 Thread Breakage

Sewing Machine Thread Keeps Breaking

http://www.servicethread.com/blog/stop-industrial-sewing-thread-breaks

Why Does My Thread Keep Breaking While Sew

Singer Sewing Machine Breaks Thread

Threading the BERNINA 880e 830e 820 Bernina Sewing Center Tampa

http://www.manualslib.com/manual/726261/Bernina-820.html?page=30

Bernina 830 880 Needle Threader Cleaning

Install a brand new needle. Verify that it is installed correctly and is facing straight forward. If it is still not threading properly, then something may be bent or out of place in the threader.

...

Skipped stitches

1. The needle is not inserted correctly

Remove needle & reinsert needle( flat side towards the back of machine)

2. The needle is damaged

replace with new needle

3. The wrong size needle is being used

Choose a needle to suit the thread & fabric

4. The foot not attached correctly

Check & attach correctly

12 wt thread generally will not work well in the bobbin...


"Aurifil 12wt Thread
Aurifil 12 wt thread is ideal for hand quilting, big stitch hand quilting and machine quilting. 12 weight thread creates a more defined and often primitive look. It's great for designs that use a longer stitch length if using a machine. When machine quilting with 12 wt thread, use 40 wt in the bobbin, increase your stitch length, lower your tension, use Topstitch size 90 needles and stitch slowly."

Sounds like the thread sensor is dirty so it is not reading the thread correctly.

There is special thread for denim/jeans. It usually comes in shades of brown. It's a little heavier and might help with the breakage. You might also want to try a little less tension. I assume you already have a Schmetz denim needle?

First make sure that you are using a new needle in the proper size. A needle with burrs on it will cause the bird's nest problem. Also, is the thread you are using old? My repair guy told me to keep my filled bobbins in the freezer to maintain their humidity, and that has made a world of difference in my stitching. Finally, how long has it been since you had the machine cleaned and serviced, and was it by an authorized Bernina repair shop? Bernina machines have special screws that only Bernina authorized repair people have - I discovered that when I took my machine in to someone who was not authorized only to have the timing go off in a matter of weeks. The authorized repair guy found clumps of thread that had been air blasted into the inner works of the machine and those clumps were getting caught up in the new stitching and causing the same problems you described. Good luck!