Menu disappears under fluid

I'm trying to build a sub-theme that is a fluid width (client wants the site to be full screen). When I make the layout fluid, the menu disappears entirely. When I go back to the 960 width, it's there.

What could be making it do this? I've even completely deleted my sub-theme and had the site create a new one and it's still like this. I'm attaching screenshots (they've been edited to remove the organization's name since the site is under a NDA).

Thanks for any help you can give.