I'm trying to set up a Term-Store based Managed Metadata navigation for my site. I can create as many levels as I want in the term store, but only the top-level terms show in my navigation. I have each term in the navigation set to Simple Link or Header. Whether my header is blank or has a link in it, no drop-downs appear below it when I mouse over or click on it. I have publishing feature enabled on my site- the site is a SharePoint Online (O365) site that was just migrated from SP 2010.
Per my test in my SharePoint online environment, I set up the Term-store for the managed metadata navigation.
Then if I configure the managed navigation for the site collection, the managed navigation can work fine.
And if I configure the managed navigation for the sub site, then the managed navigation cannot work.
You could check if the current site is the sub site.