An obstruction is present in the defroster nozzle.

1. Inspect the defroster nozzle outlets.
2. Remove any foreign objects.
3. Repair any loose instrument panel pads that block outlets.

The defroster nozzle outlet is damaged.

Use pliers in order to reshape the outlet flange. If you are unable to reshape the outlet flange, replace the air distributor.

The mode actuator is faulty.

1. Inspect the operation of the defroster air valve.
2. Replace a faulty mode actuator.

The defroster is not emitting sufficient heat.

Refer to Heating Insufficient