Menu dropdown getting chopped off in Header region

I currently have the main menu in the Header region along with the slideshow. On the front page the menu drops down over the slideshow:

The slideshow is set to display only on the front page, resulting in the menu not dropping down below the header region on any page without the slideshow:

I don't know what is causing this on pages that don't have the space the slideshow provides or how to resolve it.