Make sure that AppleTalk is on (the button next to Active is highlighted in the Chooser).
Make sure that the printer is on and is in the READY state for a few minutes before attempting to print.
Make sure that the printer has not been renamed since the last time it appeared in the Chooser.
If the printer is on a network with multiple zones, make sure that the correct zone is selected from the AppleTalk Zones box in the Chooser.