first i would open the hood,place your hand on the ac compressor clutch,it should spin freely with your hand,with no scraping at all,it will move easily,if its hard to turn or grinds while turning,the clutch is bad,also if the clutch is bad,it will throw the belt off,another issue is the pressure sensor,if there is a tiny leack,there will not be enough pressure on the switch to engage the clutch.did they do an evac then perform a pressure test,then leave for a while to see if the pressure has dropped at all?if your positive there are no leaks,check into the ac pressure switch.sometimes they go defective.,make sure all wire connector ends are tight,no loose connector ends,and all the wiring looks tight/sealed.another thing is checking the operation,of the climate control head unit.sometimes the saldor fractures internally,causing no power to relay over the switches.you can resolder the pins,or purchase a reman,unit.good luck.