AC

a dozen reasons, first, look for the refill valve, probe it and see if Freon shoots out under pressure, probably not , that means its got a leak somewhere , one Freon with green die and a stop leak can be tried, it may help show the leak, uv light shows this well, the stop leak in a can is like,, this will rebuild your engine in a can, if you know what I mean, though in some cases a very minor leak maybe