You might check the chopper knives, If they are worn bad, it can cause you to plug. If they are too rounded, they don't grab the straw, and pull it through. Joe
That's right, we had 2 850's and if the knives were worn down to much you filled the hole back with straw. Change the knives and your ploblem should end.
The other posts are correct. The first thing to do is replace the swinging hammers in the chopper even if they look good. I have had that problem with my 860 and prior to that a 914. If the leading edge of the swinghing hammer is worn too much it will not create the suction needed to pull the straw into the chopper. Good luck