By default, you have to click the Bootstrap Navbar to get the menu to drop down. This is by design. But if you want the sub-menus to drop down when you hover over the top-level navbar, add this code to your CSS style sheet.
@media (min-width: 768px) {
.dropdown-menu li:hover .sub-menu {visibility: visible;}
.dropdown:hover .dropdown-menu {display: block;}
}
