There are about a dozen similar questions on various sites.
Here is an answer I gave on another site specifically about Airprint compatible Brother printers.
Our Brother Airprint printer worked fine with ipad and iphone printing with iOS6, but like many others can't find the printer under iOS7. (This problem is a good reason not to upgrade.)
The problem is that the printer "goes to sleep" and iOS7/Airprint will no longer "wake it up to print."
Most exotic solution involve doing something that wakes up the printer, but then, it goes back to sleep about 3 minutes after the print job is finished.
Solution (1): Plague Brother (email, calls, letters) until they revise the firmware for your printer so that its Airprint feature is compatible with your printer.
Solution (2): Wake up the printer manually by pressing ANY button on the control panel. GO is always nice for this. It is NOT necessary to turn the printer off then on. Pressing a button will wake it up. While the printer is awake, it is findable on your iThingy.
Solution (3): Get into the printer's preferences and reset the "time till sleep" setting to manual, choosing 30 or up to 90 minutes. Then it will stay on longer. It is also possible to turn the sleep function off, but it is hidden and difficult to explain.
Solution (4): Download free app _Printer Pro Lite_ and set it up for your printer. Unless you want to, do not buy the complete program when prompted. Just reopen the app and tap on your printer name, and this should wake it up. It saves a trip from your iThingy to the printer.
New printer firmware is the only real cure. iOS7 has been out for about six months. Why is this Airprint compatible printer not yet compatible again?