The system could be low on refrigerant and/or poor or no airflow across the evaporator coil maybe it has a problem defrosting also look for a dirty evaporator. A system low on charge may show some icing on the evaporator coil, but it will normally only be at the inlet of the evaporator coil at the outlet of the metering device. The icing will normally be contained to this area as opposed to the entire coil being iced up. A completely iced evaporator is normally the result of either inadequate defrosting or little or no airflow across the coil. You can determine the cause by visually inspecting the evaporator fans to make sure they are running in the proper direction and at full speed. Then you can inspect the airflow leading to the evaporator coil to make sure it is not blocked.