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.
Back in 2016 the first filter in Bullet roaster looked like this. It quickly got full. Now the filter is much larger.
Here is the cleaning routine for some of the Bullet owners with that filter type:
Read more about fan setting under Roasting basics in the section Fan/airflow