Finicky setup there. While it should work fine, it sometimes doesn't (as you now know). Many times the 'host' Mac will show the printer, but fail to allow connection/printing from other machines if it is in Sleep mode. Other times you can't even see the printer from another computer if the host is in sleep mode. Some times another USB device conflicts with the printer for no apparent reason.
Some times the fix is to completely delete the printer from the System Prefs Printing panel. Other times simply changing the Printer Sharing settings will force it to work.
Ultimately, you would be better off plugging the printer into the USB port on the back of your Airport/Router would be the better option.
Sorry I can't be of more help. These types of issues are hard to diagnose without actually sitting in front of the machine.