Failure of an air conditioning unit to cool properly could have several causes. For instance, an older unit may not be as efficient as a newer model, or the unit may be too small for the area it is supposed to cool.
Another common reason why your air conditioner may not be cooling a home interior properly is because of faulty ductwork. Damaged, improperly sealed or badly designed ductwork can significantly effect how well an air conditioning unit works and delivers cool air; be certain to inspect the ductwork on a regular basis. Thermostat settings also have an impact on how well an air conditioning unit does its job. Fortunately, thermostat issues are almost always minor and involve easy fixes, such as mending loose wires. If the thermostat is battery operated, the problem could be as simple as the batteries being dead.
Refrigerant leaks are another common cause of air conditioning units failing to properly cool the home interior; this is particularly likely with older units. Refrigerant can also freeze when it comes into contact with moisture, and also cause blockage that prevents cool air from properly passing through the system. Household air conditioning units should be serviced on an annual basis, so that small leaks can be repaired before they become major problems.
Stalled condenser coils have a negative effect on an air conditioning unit's ability to produce cool air. Another possibility is that the unit's filters need to be replaced.