Stops playing videos after unlocking iphone



Been having a problem for a long time now. Using plex on iphone to cast workout videos and movies to chromecast. Phone eventually locks, attempt to unlock phone and 6 out of 10 times the videos/movies stops casting. We attempt to recast and it does not work, attempt to disconnect from chromecast and reconnect and the casting works but Plex app does not display what is being played, its at home screen. To fix we have to completely close out of Plex app and relaunch it. Any thoughts on why it would do this when all we wanted to do in most cases is pause the video or just unlock phone to check text message?