recharging a/c

My '00 galant ES has been giving problems with a/c system. Last month i paid $160 for a check and recharge of the system. the mechanic claimed it was fixed for good. but couple weeks later a/c stopped working again. the mechanic recharged it for me. now it stopped working again. i checked when i run the a/c the compressor doesn't even rotate. does it happen when pressure is too low? the shop had put a die but couldn't detect any leakage. now i want to recharge the system, does the compressor have to be running? any help is appreciated. thanks in advance.