A lot of factors to affect a symptom of less than icy air comming out of the vents. Among the most common a seepage of freon and thus a less than max efficient system.
For best results for the coldest temperature run your A/C system on the MAX setting. We have had some come in for a less than comfortable ac system that had the controls set on fresh-air. Its good to operate on fresh-air setting occasionaly to keep that condensation drain cleared out but you will get colder air if your car is trying to cool already conditioned air rather than the 100+ deg. air from outside. Another thing you can do to increase the comfort is making sure your cabin air filter is clean and allowing max air circulation. Some cars/trucks have these, some dont. Most of the newer foreign cars have a cabin air filter.
