It could be a lot of things. What browser are you using? I know a lot of times, hubpages usually works best with either google chrome or firefox, as most other internet explorers don't sometimes.
Or there might be something wrong with the embedding code, in which case, you can try copying and pasting the url of the video instead to see if that works.
Or, it could be the person that uploaded the video may have blocked embedding of the video altogether to where nobody can use it. Like I said, it could be a lot of things, but hopefully that helps.