While watching a movie on Plex Theatre, during the film it randomly minimises



Recently while watching various movie's on full screen on my window 10 media pc running both plex server and theatre sometimes the screen randomly minimises , it had done this in the past when an update comes in for windows an/or apple itunes, so I have switched the update to ask permission before installing , this was fine then recently this has reappeared. We may watch 2 films with no problems then on a third it suddenly minimises during the film. It is not a particular film as it seems to be very random. Any help/advice??