How to create a drop-down menu with visible child items in the top navigation

I'm trying to create a drop-down menu item on the top navigation. Although I see the drop-down arrow, the child menu items do not appear when I click it. How can I ensure the child menu items show properly?

To ensure the child menu items of a drop-down menu appear correctly in the top navigation, check that the parent menu item and all child menu items have the same access level. 

For example, if the parent menu is set to be visible to everyone but child items are restricted to logged-in users, the child items will not display. Aligning access levels between parent and child menu items should resolve this display issue.