HTML 5 Video tag not showing fullscreen button

I am trying to get a video with the video tag to include a fullscreen option in the controls. If I insert a video into a site using the following:

<video controls>
    <source src="filename.mp4 " type="video/mp4">
</video>

I do not get the full screen button.

Interestingly when you look at w3 the example on the page shows the fullscreen but when you click the Try me it does not.

Could it be because the tag is inside an frame? Is there a way around this?