Why is full screen not working in Episerver?

When publishing video in your Content Management System by copy and pasting embed codes and not inserting from plugins, the CMS (in this case Episerver) may modify the embed code after it is saved on your page. 

Cause

In many Episerver configurations the editor where you enter html embed codes (TinyMCE) will remove the "allowfullscreen" attribute from embed codes, blocking the full screen functionality of the player.

Note that this problem does not occur when using the Screen9 Plugin for Episerver.

Solution

If you are experiencing problem with full screen on videos in Episerver, please contact your systems administrator regarding either installing the free Screen9 Plugin for Episerver or allowing the "allowfullscreen" attribute for iframe embed codes in the TinyMCE editor on your Episerver.

Guide

Example of a guide on how to configure Episerver to allow the "allowfullscreen" attribute.
http://www.jondjones.com/learn-episerver-cms/episerver-developers-guide/episerver-and-tinymce/how-to-prevent-tinymce-the-richtext-editor-stripping-attributes-off-your-html-elements-in-episerver 
(This guide is not written nor maintained by Screen9 and Screen9 cannot guarantee this guide will apply to your specific Episerver installation.)