Hi Andrew, 

Thanks for reaching out to us!

Due to mobile standards Video Backgrounds are disabled on mobile more so for autoplay video backgrounds. 

There are a few ways you can get it to work however, via the use of a customization or a plugin. 

Here's a plugin you can try: https://wordpress.org/plugins/video-background/ 

Also please see Section 3.1 Troubleshooting Video Backgrounds in the user guide :http://themenectar.com/docs/salient.pdf.

And if you are using the Nectar Slider: 

http://youtu.be/FLU9Pxo_RD8?t=6m37s

Let me know if this helps. 

Cheers