Your AC is not reaching the pre-set desired temperature. This can be due to several different factors. By slowly raising the temperature you can determine the point at which the unit will cycle on and off (you will be able to hear the unit coming on and shutting off). This is your maximum achievable cooling temperature. To lower that temperature, there are several things you could do. These include adding insulation in the attic, covering windows on the east and west sides of the house, providing shade for your outside condenser, and decreasing the amount of heat generated inside the house by not running heat-producing appliances during the day.