The navbar is not in ng-bootstrap component list but if you follow ng-bootstrap recommendations, you only need to import bootstrap css file. Therefore the navbar collapse and dropdown don’t seem to work.
Link to minimally-working StackBlitz (plunker) that reproduces the issue:
In this example, the collapse doesn’t work for small frame and if you increase the size of the frame, you’ll see that the dropdown doesn’t work either.
Versions of Angular, ng-bootstrap and Bootstrap: