Thanks for the speedy reply. I'm definitely using this plugin and it is activated. I just updated to 1.2.3 and problem still exists.
I see what you're saying about the code using a different class.
Although the YouTube embed is like this…
<span class='embed-youtube' style='text-align:center; display: block;'><iframe class='youtube-player' type='text/html' width='912' height='543' src='http://www.youtube.com/embed/u0K-efbwB3w?version=3&rel=1&fs=1&showsearch=0&showinfo=1&iv_load_policy=1&wmode=transparent' frameborder='0'></iframe></span>
Vimeo is like this…
<div class="rve" data-content-width="912"><iframe src="//player.vimeo.com/video/78819312" width="912" height="513" frameborder="0" title="X-PARTICLES 2.5 EMITTER" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe></div>
Which appears correct. Any idea what might be causing the difference with YouTube?
I'll try disabling all other plugins and see if it is a conflict.
Thanks again for your help.