When you add a 6th navigation item in the Quick Launch bar, the element is displayed neither in the settings page in the navigation.
However, if you delete one of the five elements, the object appears in front of it created in the navigation.
When you first create an app (custom list Document Library, etc ..) will create a new main navigation element. This element is called "Last Use" or "Recent" in english. Unfortunately, this element the property that allows only 5 sub-items.
It is recommended to remove this upper point in the navigation and to replace it with a custom About point.
Often this point is renamed when creating the template and therefore is not directly identifiable!