Sub menu highlighting

My header menu has 4 buttons, which are highlighted when on the page (yay).
One of those menu buttons has a sub menu (also yay).
When on the parent page or any child page, the whole sub menu is highlighted (not yay).

I can’t link my site, as I’m doing this offline, but here’s the (hopefully) relevant code:

nav ul li.current-menu-item a:link,
nav ul li.current-menu-item a:visited,
nav ul li.current-page-ancestor a:link,
nav ul li.current-page-ancestor a:visited {
  background-color: #006ec3;
  color: #FFF;
}

I’ve tried messing around with the code, including using nav ul li.menu-item ul li.current-menu-item a, but nothing I’m doing is working. I already spent way too long getting the sub menu to work properly, and now I’m getting frustrated at the CSS not playing ball.