Central air conditioning is great but only if it works. Central air circulates cool air through ducts in the walls, ceiling and floors to cool the entire house rather than a single room, as a window air conditioner does. Air conditioning also helps dehumidify the air, making it more comfortable to be indoors during a muggy, summer day. If the central air conditioning unit isn't cooling, though, all you're doing is wasting energy.
Listen to see if the air conditioner is always running. If so, it can indicate that the air conditioning system is undersize and unable to cool your home properly. This is usually not the problem with central air, but if the system was retrofitted to the house, it could be a reason. It can also be an indicator that the difference in outdoor vs. indoor temperature is just too great for the air conditioning system to cool.
Feel the airflow coming from the air conditioning vents. If the air flow is weak, the ventilator fan might be failing or ducts could be blocked. Other possible explanations could be damaged ducts or a need to switch your air conditioning from the winter to summer setting. If the airflow is fine and cold, then the problem could be poor insulation in the home.
Remove a few heating registers. If you can see dust, it is an indication that the ducts probably need to be cleaned. Even if it is not the problem, proper cleaning will help your air conditioner run more efficiently.
Cut the power to the air conditioner. You'll need to check your breaker panel and flip the switch that controls the central air to the off position.
Clear debris from inside and around the outdoor condenser. Central air conditioning pulls air in from outside with the fans inside the compressor. It's also easy for leaves and small branches to be attracted to the compressor. While they usually can't get inside the compressor, they can block it. You'll need to remove the protective grill and remove any debris that did get inside. Use a soft brush and clean off the fan blades.
Turn the power back on and reset the air conditioner. Let the system run for awhile and see if there is any change. If not, you will probably need to get a professional to come to your house and look at the system.