This could be due to insufficient refrigerant or lack of servicing.
If your air conditioner is low on refrigerant, either it was undercharged at installation or it is leaking.If it is leaking, simply adding refrigerant is not a solution. A pressure test to determine the leak should be conducted, fix the leak and then charge the system with the correct amount of refrigerant.
If it's due to the latter, general cleaning is needed to improve the performance of the air conditioner.