There are a few reasons that fullscreen may be disabled for a visitor to your page. If you have determined that the problem is not due to browser permissions of browser add-ons, you can check the embed code for the video and add "allowfullscreen" to the iframe tag.

This would look something like this:

<iframe width="853" height="480" src="http://www.youtube.com/embed/XXXXXXXXXXXX" frameborder="0" allowfullscreen></iframe>