We don’t find any problem using this from the Example Code:
<div class="ytviditem"><iframe width="1400" height="778.5" src="http://www.youtube.com/embed/m6wHSyKS8Tw?autoplay=1&loop=1&playlist=m6wHSyKS8Tw&showinfo=0&controls=0&mute=1" frameborder="0"></iframe></div>
Our Testing environment is showing the video with autoplay without any issue.
You should change the file name m6wHSyKS8Tw after embed/ and after playlist=