The answer here is twofold. It is possible that the system has a small leak, and now is low on refrigerant. This will allow the system to blow cold, but just not as cold as it has before. In this case, check the low-side pressure and verify if the system is low on refrigerant. If it is low on refrigerant, add refrigerant or a refrigerant with a Stop Leak to the correct pressure.
If your pressure is correct, adding additional refrigerant will not make the system blow out colder air, but will, in fact, cause the system to become overcharged. This causes the system to work less efficiently and will result in warmer air blowing from the vents.