A/C wont blow cold air. on 1999 Volkswagen Jetta

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When I turn on the a/c it blows cool air but not like it use too. I had it charged last year and when I engage it I can hear the compressor click. Then I turn it off and I can hear it disengage. It is starting to get hot and I have the kids with me most of the time.
(2) Answers
It is low on refrigerant. Globalhelper is right. Don't try topping the system off yourself or a friend who is not a trained mechanic.
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