Angular 4/5 Bootstrap 4 Navbar NgbModule

I'm trying to create a navbar dropdown when the window is halved in size. The dropdown doesn't work at window width 950px, however it works at window width 768px.

navbar.component.html

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" (click)="toggleMenu()">
    <span class="navbar-toggler-icon"></span>
  </button>

  <div class="collapse navbar-collapse" id="navbarNav" [ngbCollapse]="isCollapsed">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Link</a>
      </li>
      <li class="nav-item dropdown">
        <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
          Dropdown
        </a>
        <div class="dropdown-menu" aria-labelledby="navbarDropdown">
          <a class="dropdown-item" href="#">Action</a>
          <a class="dropdown-item" href="#">Another action</a>
          <div class="dropdown-divider"></div>
          <a class="dropdown-item" href="#">Something else here</a>
        </div>
      </li>
      <li class="nav-item">
        <a class="nav-link disabled" href="#">Disabled</a>
      </li>
    </ul>
  </div>
</nav>

navbar.component.ts

import { Component } from '@angular/core';
import { NgbModule } from '@ng-bootstrap/ng-bootstrap';

@Component({
  selector: 'app-navbar',
  templateUrl: './navbar.component.html'
})
export class NavbarComponent {
  public isCollapsed = true;

 toggleMenu() {
    this.isCollapsed = !this.isCollapsed;
  }
}

At 950px the dropdown doesn't hide the menu items. However, if I uncheck 'display: block !important' for class '.navbar-collapse.collapse' then it works.

At 768px the dropdown starts to work (it hides the menu items) and the dropdown button moves over to the left.