Diagnosis
The most common cause is the damper not opening. The exterior damper flap (on the wall cap or roof cap) is stuck closed or the internal duct direction is set wrong.
How to Fix It
Go outside and check the exterior wall or roof cap. Ensure the damper flap moves freely. Inside, check the duct configuration. On this model, a damper actuator inside the top compartment controls internal flaps. Make sure it's connected and moving when the fan is on.
If That Didn't Work
The blower wheel itself might be caked with grease. Remove the vent grille and clean the blower wheel and housing thoroughly with degreaser.
Heads Up
If venting outside, the total duct length should not exceed the limit in the installation specs. Too many elbows or long runs kill airflow.