My mechanic is always replacing a certain part in VAG cars AC systems. Can't remember exactly but a pressure switch or diverter valve? Apparently it cracks and slowly leaks the gas. He said it's like 90% of the time responsible for cool, but not icy AC.
Mind you, in both my old A3 and my current A4 the AC never gets icy once the ambient temp is over 35. My dads mid 90s Toyota corolla however, well, that a basically a fridge. And it's never had an AC regas or service.