Modern browsers have started to block videos from automatically starting unless the video is muted, so in this case the user needs to manually start playing the video.
Videos is set to automatically appear in full-screen mode and to have audio on. Depending on your browser it might play the video, or it might just show you the "buffering" icon since the video is not playing.
Fortunately, you can usually override this. If you float your mouse down to the bottom of the screen you should find the video controls pop up and you can just hit the play button.
We know this is inconvenient, but most Safari videos are not impacted because you can see the entire player and it's obvious you can start playing the video. It's just the videos which are linked to from a page, and most books don't have those.