Hi Geert & Idmarkely,
I'd searched for this answer last fall & wondered why it also wasn't working for me, when it works for David & Chris. Just this past week I figured it out.
When I upload my videos, I've always checked the box "autoplay audio/media" because I wanted to make sure students saw the video before pressing the "continue" button at the bottom of the plaque. When the "autoplay audio/media" is checked the video will not go to fullscreen nor will it rotate. However, the videos WILL go to fullscreen & rotate if we do not check this box when uploading our videos into our game media. The plaque loads the video with a grey screen containing a play button in the centre.
As an added precaution, so students won't press the "continue" at the bottom of the plaque before viewing the video (since many of my game elements are set to a "locks" trigger "player has not yet viewed plaque"), in the text of the plaque I have put <b>Press the play button </b>above to watch the game tutorial/video/ etc.