If HTML5 Video isn’t working correctly, it’s possible your server doesn’t support all HTML5 file-types yet.
Here’s how you can update your server to support HTML5 Video file-types.
Make sure you included all needed video sources. Different browsers need different file formats for playback. An overview is available here.
Please test if the video will playback in the browser when putting the file`s URL in your browser’s URL bar.
If the video plays back and still does not work through the WP plugin please install the WP Htaccess Editor plugin and add the following lines on top of the htaccess file (please be careful with this file since any error in this file may cause malfunction of your site!):