The first thing to check is the air filter in your furnace which is use in conjunction with the central air. Air filters are located in various locations depending on your furnace. To find your air filter, look for the vent shaft that goes into your 
furnace. There is typically some type of a door that slides or opens on the shaft. When you open that door, brace yourself: it could look pretty dirty. Air filters come in different sizes so you will need to look on the old filters cardboard sides, you will need the filter's size dimensions to know what size filter you need to purchase. You may want to buy several as they do need to be replaced every one to three months depending on which air filters you buy. When installing air filters, locate the small arrow and the words AIR INTAKE on the air filter. With the arrow pointing IN the cavity, install the air filter and close the air filter intake door. Running your central air with a clogged air filter will not only cause your central air to not function properly, it can cause the unit to burn out completely and may require you to have to replace the central air so make sure you change your filter regularly.
If the problem persists you may have dirty coils or a restricted condenser which would require a central air trained professional.
