90 % of the time it is just dirty, although the other answers can cause this problem. i would have it cleaned before anything else.
You either have a dirty air filter or the unit is leaking refrigerant and is undercharged.
the first answer is the best bet, but also make sure the out side coils are not covered up with vines.
the first two answers are correct. your unit my be too small for the area you are trying to cool. I have had customers try to cool entire homes with 1 5,000 btu window unit.