AirPrint wouldn't work until I enabled IPv6 on my printer
I have a Brother MFC-J285DW printer, iPad Air 2, and iPhone 6. Recently my AirPrint app stopped finding my printer. Sometimes my iPhone worked, but the iPad hardly ever. My Brother printer app always worked. I had trouble shooting sessions with both Apple phone support and at the local stores Genius Bar. They really found nothing wrong.
I Did some looking around and found a recent post about enabling IPv6 on the printer. Since IPv6 wasn't enabled I did that. Now AirPrint is working like a charm. I hope this helps someone.
iPad Air, iOS 8.4