The screen shouldn't turn, just the chopper blade, also it would be good to pull the chopper/screen assembly out and check the motor impeller were the chopper/screen assembly connects to, and make sure that plastic piece is stripped.
Here's the repair manual for your model:
Page 17 Read 1-4 where it says this:
SERVICING COMPONENTS INSIDE THE WASH TUB
Removing the Inner Feed Tube and
Lower Spray Arm
Then on Page 21 it starts to get into detail of how to access the rest where it says this:
Accessing Components of the Sump Assembly
It shows how to remove the chopper screen.