Twitter Bootstrap：下拉菜单在移动视图中消失

Question

我设法创建的导航栏工作得很好。但是，当我通过重新调整窗口大小来折叠导航栏时，它会在我尝试打开该视图中的菜单时消失。 为什么下拉菜单在移动视图中消失。我做错了什么？

HTML code：

 <nav id="mainNav" class="navbar navbar-default navbar-fixed-top">
  <div class="container-fluid">
    <!-- Brand and toggle get grouped for better mobile display -->
    <div class="navbar-header">
      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
        <span class="sr-only">Toggle navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
      <a class="navbar-brand page-scroll" href="#page-top">Kirori Mal College</a>
    </div>

    <!-- Collect the nav links, forms, and other content for toggling -->
    <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
      <ul class="nav navbar-nav navbar-right">  

       <li class="dropdown">
          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-expanded="true">Why Recruit?  <span class="caret"></span></a>
          <ul class="dropdown-menu" role="menu">
            <li><a href="#">Why Recruit?</a></li>
            <li><a href="#">Demographics</a></li>
            <li><a href="#">Faculty</a></li>
            <li><a href="#">Academics</a></li>
            <li><a href="#">Alumni</a></li>
          </ul>
        </li>
 <!-- End of nav bar tabs -->
      </ul>
    </div>
    <!-- /.navbar-collapse -->
  </div>
  <!-- /.container-fluid -->
</nav>

链接到Website和 链接到CSS file。