Typically, this means one of two things 1) Low airflow. This could be dirty filters, closed registers, undersized ductwork, blocked register by furniture or carpeting. 2) Low on refrigerant. This means you have a freon leak. You should never have to top-off or add freon to an air conditioner. You never add freon to your refrigerator and you’re a/c system is no different. If you add freon, you have a leak.