I had that problem this year and it was because the beater was taking all the straw away for the rotors right in the middle. therefore throwing it all into the middle of the chopper 2 little deflectors on the grate under the beater did the trick. also check your wind knives on the outside of your chopper. they could be worn off.
maybe your feeding one rotor more then the other check the first deflectors on the auger in front of the rotor. they should be set as far out as possible. But if its puttin stuff thru good that isnt the problem. now wind knives worn more on one side then the other would do it. I made the deflectors myself and had to drill holes in the grate so you would have to too.