Check to see that feed chains are not out of self tention range. Check to see that serrated feed chain slats are not rounded off bad and that they are as aggresive of original equipment slats if not original equipment. Check for large number of bent slats or one side of chain looser than the other. Rear feeder drum shocks have helped the transition between chains greatly. Poor condition cylinder bars and concave have been the culprit of dust out feeder for as long as I can remember. like Rolf said if it isn't traveling threw rotor well you could get dust problem. Helicals wore worse on one machine more than the other. Chopper blades may need to be turned or replaced. Does one machine have chopper and the other notIJ Check out the upgrades like shimed helicals or double stacked both with extra helicals on thresher side. Maybe they would prefer the channel helical kits.