I think it has a slow leak.
I have owned the car for four years and never really used the A.C. It had a few electrical issues that I resolved and the A.C. started to work, but only blew cool. I checked the pressures before I added the 12 oz freon and it was reading 10 psi low side and 180 psi high side @ 2500 rpm and 80 F. Figured if it held a charge for over four years, I'd give it a try without pulling it apart and spending some $$$$. Some lose is expected over four+ years?