Non-functioning embed code is often due to a CMS not allowing <script> tags, or a WYSIWYG visual editor breaking the embed code.

 

Wordpress:

Wordpress.com doesn't allow <script> tags (but self-hosted wordpress sites do).  Many self-hosted wordpress sites have a WYSIWYG visual editor enabled for posts, but you'll need to make sure that the plain html editor is enabled, and that someone doesn't come in and accidentally open the post w/ the visual editor, which will break the embed code.

 

Alternatively, you could try using the Amara auto-embed script, which will add the video-widget to every video on a page.