According to HowStuffWorks, there are several reasons why an air conditioner blows cold air but does not cool a room. Cleaning part of the unit is sometimes necessary to address the problem. It is possible that a part is malfunctioning or missing. It is also possible that the unit is too small for the room, that the space lacks sufficient insulation or that the thermostat is set too high.
HowStuffWorks recommends first checking that the temperature of the thermostat is set properly. Then check both the filter and the cooling coils for dirt if a unit is not providing sufficient cooling. A dirty air filter in the unit impedes air flow through the cooling coils and prevents the cool air from exchanging with the warmer air in the room.
About.com notes that portable air conditioners require a duct to expel hot air outside the cooled space. If this duct is leaky or missing, the unit blows both cool and hot air from the front of the unit instead of funneling the heat outside. With correct installation, these units help to cool a stubbornly hot room.
According to the Department of Energy, ducts for a central HVAC system develop leaks. In a home where one room remains hot while the rest of the house is cooled, it is important to inspect ducts for leaks and make any necessary repairs. It is also important to make sure that the air conditioner is rated high enough for the space that needs cooling. Both the size of a room and adverse conditions, such as heat from cooking appliances or insufficient insulation, affect how well a unit cools a room.