In a 2015 Subaru Impreza, when the A/C compressor is powered on, the two middle vents blow air that was obviously cooled. However, the remaining vents (one on dashboard near drivers' door, one on dashboard near passenger's door) blow air that is not cooled - the air temperature does not change whether the compressor is on or not.
Note that the airflow volume and velocity feels the same across all four vents, and all four vents do change airflow volume and velocity when the fan is moved through all its power settings. The heating settings seem to work as expected, as does the defroster setting.
What might cause this? If there were a blockage or a vent channel leak, I would not expect it to affect multiple vents. And I would expect a blockage or vent channel leak to affect the volume of air, but not the temperature.