Check the pigtail to the compressor for 12v with the air on. If you have power but no operation, you've got a faulty clutch.
If you are certain it has a good charge (I'm wondering how you can tell if it doesn't work, please advise) you could try a jumper in the low pressure switch and see if that will get it to engage. If it cools good like that, the low pressure switch is faulty.
I tell you what, on my own stuff, I'll run 12v straight to the compressor and force it to run before I'll drive around without A/C.
With a test light, make sure the compressor pigtail has a good ground as well.