It’s not immediately obvious unless you’ve run into this before, but the dropdown nav menu feature of Bootstrap requires both jQuery and the bootstrap.js includes. Maybe because I normally don’t use any of the JavaScript features of Bootstrap I haven’t run into this before. It is covered in the install docs here as a requirement, but maybe I’ve not noticed this before. It must be a common issue though, here’s a question on SO.
If you leave out either of these the menu renders as expected but the dropdown doesn’t do anything. Here’s a plunkr with both .js files included as a working example (example HTML from the example here).