when then a/c is on do you have an earth and a live at the compressor? If you don't have both then either the pressure switch is faulty, or there isn't the correct pressure in the system. Either the live path or earth path is via the pressure switch. This is to ensure the system won't over run (to much high pressure). Or run dry ( no gas) . A basic test with 2 pressure gauges at a decent garage should be able to give you the low down