2006 Suzuki Reno AC problem

Good day,
cooling is very week. In summer it is like 40% of the normal AC cooling and at night 70%. I went to the mechanic, he told me that every thing is fine except compressor that needs change. I got a used one same car model and he fixed it and filled gas and I paid him( 140$ ). He was parking my car in front of his garage in the sun for about two days and when I checked the AC he told me just because of hot climate but when you drive it will cool but unfortunately the problem has not been solved. I went to same mechanic after two month and he checked whether there is leakage or not but there was no leakage and he told me just we will fill it with gas (30$ ) and It will be fine but same problem still. Only cool at night. Till now I do not know what is the problem.

Regards