Every 3 to 4 months I go to shop to add refrigerant shop can’t find leaks more than a year doing this .what is the possible cause

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After 2 months again it blows hot air or it doesn’t blow call air anymore ,you can’t hear any sound or any engine sound change.Reason I ask cause this car I bought here in USA but no longer here . I want to buy parts before going home to other country where no Lexus dealer there.