Parallax not working – OMNI theme

This theme (OMNI) should have parallax scrolling background images, but they appear to be fixed in place and don’t scroll.

I’ve had this working when i set the theme up on a different domain, and here’s how it should work as seen on the demo page for the OMNI theme https://themeforest.net/item/omni-responsive-one-multi-page-html5-parallax/full_screen_preview/5082172

hope someone can help.