Navigation Gotcha in SharePoint 2013
Lots of people like to use Navigation to create dropdowns on their top link bars.
In SharePoint 2013, this functionality is affected by the Site Layout of the theme you choose.
If you choose Oslo, it changes the menu structure of the site and your custom navigation won’t work. You need to leave it on Seattle for it to work.
Oslo hides the Quick Launch and moves that nav to the top instead.