Here, I'll show you.
Click here, and the header appears just fine:
But click within a thread, and the header disappears. Scroll to the bottom, though, and you'll see it at the bottom of the site!
Any ideas what's causing this? The only thing I did today, of substance, was create a child style from the main style so that I could make the child style responsive and the main one not.
Oh, and when you're logged in, this issue doesn't happen.