Nav bar not showing open menu

I was following the nav bar tutorial video and got as far and styling the open menu but I can’t get the open menu to show up - either in design view or in a live preview. The hamburger icon simply gets a grey background and nothing happens. Any ideas?

Well, I went ahead and built it from scratch the same way again and this time it works just fine. Must have been a glitch. It was doing it in a nav bar on one of my other sites too. Weird.