I've had the same problem here in Dec 2013.
Epson XP-400 does not show up as an Airprint available device more than once. Network is fine, printer can be used from Windows the whole time, but sometimes it's found sometimes it's not.
I don't have the answer, but the Firmware on the Printer is fully up to date and even the App from Epson "EPSON Prtiner Finder", only finds it once and a whilte. I can usually find it by restarting the printer, but as people here have indicated, only once and then it does show up on the iPad or iPhone, OR, the Epson the Finder.
So, my vote is to a bug with the Epson software and I guess I have to find a more reliable Airprint printer.
SO SillieMe and Joel. It's not your imagination and you had everything configured right...