I recommend cleaning he bag using mild soap and water, and use a mild bristle brush to scrub the inside of the bag. Bags can accumulate stuff on the inside that will prevent good airflow. The green grass is being propelled by it's greater weight so it picks up. The dead stuff needs to be propelled by airflow and that appears to be your issue. I just used a HRX217 to pickup debris from dethatching my yard and it picked up just fine. When I run my mower, I can see the bag bulge outward when the blades are running. Looking at the top of the bag, it will bulge from airflow, then it will sag if I try bagging too much and the chute into the bag clogs up due to the bag being full.