Check to determine if box is reducing too far. Evaluate box minimum setting. Conditioned Space Is Too Cold Supply air temperature is too cold. Too much supply air. Diffuser pattern or throw is incorrect causing drafts.
Temperature sensor is located incorrectly or needs calibration.
Conditioned Space Is Too Warm. Supply air temperature setting is too warm. Not enough supply air. Refrigeration system not operating properly. Fan-coil evaporator is iced over because of low airflow.