Why won't my embedded videos play after I reopen powerpoint after saving

I have just bought a new MacbookAir(Tuesday)

I do a lot of powerpoint presentations for work and have now compiled one on my Macbook.

However when I embed the videos into the presentation they are fine while the presentation is ongoing.

As soon as I shut it or save it, once I reopen it, it does not play the videos at all. The screenshot is there but no videos.

The videos are MP4 format downloaded from YouTube.

The same format and videos worked with my old PC

So just wondered if anyone knows what I am doing wrong?

Many thanks