Are you running small wire concaves? Worn rotor bars and improperly leveled concaves can be a big factor. What are you running for shoe seive? If you don't have short finger grain shoe, it will be very hard to send much of anything back up the tailings elevator. Even feeding and the amount of straw you are taking in can really change how a combine works too. What kind of yeilds, moisture are you dealing with? Are you running the combine to capacity? A kill stall would tell you what is going on inside you're machine. There are a lot of variables to take into consideration.