Its not only the fan setting that determins the airflow in your roast batch. Also the exit passage of the air makes a difference.
→ If it flows easily the airflow is bigger.
→ If the exit passage is somewhat restricted; the same fan setting will give less airflow.
If the filter in the chaff collector is dirty (full of chaff and coffee oils) the passage is restricted. There for regularly cleaning is a good idea.
See what routine Bullet owners got in the Bullet guide under Fan/airflow