Running a Imac with Sierra 10.12.13 and Itunes 188.8.131.52. Can't get Airplay to work outside of Itunes. Playing a movie through Itunes; it works great. Playing a movie through Beamer; it works great. Using Airplay; all I get is sound and no video. I have rebooted the Imac; and the modem. No luck. Any solution to this?