There is an issue in that AirPlay of video from iOS 9 devices is not compatible with all versions of ATV2 firmware except the latest (6.1.3). Your jailbroken ATV is probably on 5.3 firmware. There is a beta of a 6.1.3 jailbreak but it is tethered jailbreak and can only be run from a Mac at the moment. Note also that updating the firmware on a jailbroken system wipes all existing content during the process.