a few quick things that come to my mind when addressing AC is: too much freon, bad elctro. clutch, unseen leak, bad compressor, and i heard that if moisture enters it can affect the ac.
first research online a bit i dont know for sure what the pressure should be. but once you find out if its correct or not fix that issue.
if it is correct then to test the clutch is easy on the ac compressor there is a little plate towards the fender side of the pulley that is electronically magnatized and then in turn acts as a clutch and engages the ac pistons inside the compressor
so to test the clutch go and turn the car on and turn on the ac. then go under the hood and see if the clutch is spinning too with the pulleys or not. if the pressure is right and the clutch is not engaging it can either be to far gapped or have a bad clutch if your are for sure the clutch is not spinning you can stick a screwdriver near it and if should pull the screwdriver to it if its good if it doesnt its bad.
ok on with the compressor if the pressure is correct and you can see the clutch spinning in unison with the pulley and belt then it is probably an issue inside the compressor there are plenty of seals that can go bad inside of one.
oh by the way an easy way to rule out leaks is getting some UV dye from the auto store and pour it in then get a UV light and glasses and any leak shows up flourescent pink or yellow depending on what brand you get
so long story short in order of importance:
add the dye and make life easier on yourself
make sure the pressure is correct
then check the clutch for full engagement into the pulley assembly
then if all the others are good i would replace the compressor