Why does the AC not blow cold air at stoplights?

How was the system charged initially ? Evacuated , then after new comp. installed (with appropriate amount of PAG oil) was the system vacuumed down for at least 30 minutes? Then charged with a calibrated machine to capacity spec(I believe .55Kg)? The reason I ask this first , is because if the system is even .05Kg low , it can have those symptoms. Also do the under hood fans both come on with A/C switched on? If only one is coming on , that issue needs to be diagnosed as well, it too may be causing performance problem.