Air Conditioning problem
2002 Ford Falcon Automatic

I had a ford falcon and its aircon is not working properly. Whenever in park position it works and whenever I drive it or car is moving it works But on traffic lights as soon as it stops it stops working. It means whenever in drive mode and car is stationary aircon doesnt work at all.Its gas had been refilled but had same problem . please help me as already shown to many mechanics but they are of no help to my car problem