导航栏下拉按钮在twitter bootstrap中不起作用

Question

我对twitter-bootstrap感到很新，我的网站在桌面等大屏幕上运行良好，但是当我调整浏览器大小以影响手机屏幕时，它并没有显示我的图标（下拉列表）允许我们选择导航栏。

HTML                              

      <div class="row nav-menu">



        <a href="/">
          <div class="col-md-2 col-sm-3 columns">
              <img class="logo logo-light logo-wide" alt="Logo" src="img/logo_t.png">
              <img class="logo logo-dark logo-wide" alt="Logo" src="img/logo_t.png">
          </div>
        </a>

          <div class="nav-header">
              <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#navigationbar">
               <span class="icon-bar">About</span>
               <span class="icon-bar">Technology</span>
               <span class="icon-bar has-dropdown">API</span>
               <span class="icon-bar">News</span>
               <span class="icon-bar">Jobs</span>
               <span class="icon-bar">Contacts</span>
            </button>

          </div>

           <div class="col-md-10 col-sm-9 columns ">
            <ul class="menu navigationbar">
            <li><a href="/about">About</a></li>
            <li><a href="/technology">Technology</a></li>
            <li class="has-dropdown">
                <a href="#">API <span class="glyphicon glyphicon-chevron-down"></span></a>
              <ul class="subnav">
                <li><a href="/api">Plans</a></li>
                <li><a href="">Documentation</a></li>
                <li><a href="">Login</a></li>
              </ul>

            </li>
            <li><a href="/news">News</a></li>
            <li><a href="/jobs">Jobs</a></li>
            <li><a href="/contact">Contact</a></li>
          </ul>
        </div>

      </div>


       <div class="mobile-toggle">



      </div>

    </div>
  </nav>
</div>  <!-- navbar end -->

请帮助，谢谢。