Bernina 770qe auto thread cutter isn't working. I think it may be cutting only part way through the thread. Sometimes thread breaks at fabric if pull the fabric, mostly doesn't cut at all.

SOURCE: Bobbin Thread cutter no longer works

I have a designer SE, I ran across this site and found it very helpful. This person is a retired machine repair person and has a wealth of knowlege. <http://www.sewmuch4retirement.com/tips.html> from what I understand the blade needs to be cleaned as part of the Maintenance, it also can be replaced.

Take a look at this site, perhaps it will help

Far Away Grandma 

SOURCE: Sewing thick fabric and machine won't send it through

Sergers are not really meant to sew very thick material. The blades have a maximum amount of range, or thickness that they can reach to cut. If you go outside that range, get ready for a big repair bill! You can break loopers and even the needle plate when you "push" a serger. My advise would be to use you sewing machine for the, "pounding" work. The type of material matters too, A thick Quilt like fabric can be sewn becatuse it's not dense material. A thinner harder denim could max out at 3 layers because of it"s hard finish. If the fabric you're sewing isn't dense like, you could just need new knives.

SOURCE: The needle thread on my bernina minimatic keeps breaking.

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?

SOURCE: pfaff quilt expression 4 It shreds

Are you using the right needles, size and type? Before, you wreck your material, take it in and have it serviced. I know everyone hates to pay for service, but if you like your machine it will be worth it to have it running at its peak. Just like a car, we spend tons on them but expect a sewing machine to never need anything. Remember, it is worth it.

SOURCE: sewing machine motor jams when I sew thick fabric

a part of the computer is malfuctioning, & needs to be checked
try a new needle
rethread the machine using every thread guide
clean under the needle plate & in the bobbin area for loose threads & lint