Big Noob: Don't know my element

It looks like you don’t need my plugin at all, since the header is already at the top of the screen (and so it always needs to be sticky, not just when you scroll).

The reason why it’s not sticky on your individual posts, is because of this rule in your CSS:

#header.abs {
    position: relative;
    z-index: 999;
}

I’m not sure what it’s supposed to do, but the fact that it has a position:relative (overriding the position:fixed) is why it’s not sticky.