Clogged, stuck, or full of ice from air in the system.
Any time the system leaks down, air and water will diffuse in throught the leak while the freon is going out -- I know it doesn't sound possible, and for really big leaks it's not that much, but the air and water will diffuse through the boundry layer along the metal or rubber of the fitting (or whatever else) that is leaking.
Always a bad idea just to blow more freon into a low system without attempting to locate the leak.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!