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.
Temperature sensor is located incorrectly or needs calibration.
Low pressure duct leaking. Low pressure duct not insulated. Static pressure in the system is too high. Diffuser is too small. Not enough static pressure at box inlet for proper operation.