The most likely cause of the symptom you describe would be because the system is low on refrigerant. Most AC systems are designed to shut the compressor off to prevent damage if teh refrigerant level drops too low. A sign of this would be a compressor clutch cycling on and off fairly often, or not coming on at all.
It is normal for an AC system to lose a few ounces of refrigerant a year through seepage, especially as the car gets older, so if you have not had it recharged recently then this is likely your problem...
If you take it to a technician to be recharged, as part of the service he will make sure tht there is no air trapped in the system, make sure the cooling fans are operating correctly, and test for leaks.
Attempting to add refrigerant at home is not a recommended practice; on smaller modern vehicles such as yours, it is absolutely critical that the charge level be within an ounce or so of specs, or the system will not perform as designed. There is no way to measure from a 14 ounce can, a technician uses a digital scale to measure the exact charge going into the car.
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