I’m not sure what it could be, but there isn’t anything inside the Epik theme itself that would affect or prevent menus/submenus from showing up. The only file in the theme that could possibly affect the menus would be the functions.php file, and if you take a look at that file you’ll see there isn’t anything that would cause an issue. I’m also working on a site for a client that uses this plugin and it works fine.
Since it is a 3rd party plugin, I would recommend contacting the Uber Menu Developers to figure out what’s going on.