I use the “mega-menu” css for my live site (ethicalstl.org). I cloned the live site (on SiteGround). However, the mega-menu css has no effect on my clone. In examining the produced code the menu items all had “fl-has-submenu mega-menu-disabled”
An update. I discovered that, while I was inspecting the source code of the published page it had gone into it’s small device mode. That was the cause of the fl-has-submenu mega-menu-disabled
When I expanded the view to see the large site that setting was not present. So, that is not the cause of the problem.