First, I see printers listed here in this thread that are not AirPrint printers:
http://support.apple.com/kb/HT4356
In those cases, are you using some sort of AirPrint Server Hack? Which one?
If you are using an AirPrint printer, it must be on the same subnet and Bonjour packets must route. So get the WiFi IP address of your iPhone (iPad, iPod) and the IP address of your printers. Compair them.
As for Bonjour routing, try this Bonjour Browser App (https://itunes.apple.com/us/app/discovery-bonjour-browser/id305441017?mt=8) Does it see your printers?
Finally, if your printer is on a different network (HP Internal...), you will have to re-select the printer everytime. There is an iOS7 bug where it does not rememeber your most recently used printer.