Primary link tabs don't stay highlighted on child pages

I have a primary menu with the normal tabs at the top. A few of my menu items have dropdown menus, and the child items also show up in the orange bar. That all works fine.

The problem is, the primary link tab only stays highlighted when you click the first child menu item listed under it. If you go to any other pages listed under that same primary link, the tab is not highlighted, and you have no way of knowing where you are in the menu system.

How do I get the parent link tab to stay highlighted on all the child pages?

I'm using a new subtheme, where I've taken almost everything straight from Marinelli with very few changes. Besides, the problem remains even when I switch back to Marinelli.

I've searched both open and closed issues, and couldn't find an answer but if it's already answered, I'd appreciate a nudge in the right direction.

Thanks!