Printer Designator Disappears


I have OS 10.4.2 and an Apple Airport.
Sometimes when I hit File + Print I am happily offered a choice of printers which includes "Shared Printers". Other times this option is gone and I just have the choice of either my named (connect with USB) printers or of Bonjour (?) printers.
However, if I want to print wirelessly, only the "shared printer" option will allow it. Anything else just gets lots of red/stop/exclamation points.
My question is, how come this option of "shared printers' diappears, and how can I assure it is offered all of the time??
If there are no shared printers to be found, the option will not present itself. Is the shared printer on-line and being shared all the time? Where is the printer being shared from?
make sure you are not inadvertently jumping on another wireless network that may have the same name. This will cause you to not be able to access your shared printers.