为什么代码一样，可是不能实现效果

       <div id="carousel-example-generic" class="carousel slide" data-ride="carousel">

          <ol class="carousel-indicators">

            <li data-target="#carousel-example-generic" data-slide-to="0" class="active"></li>

            <li data-target="#carousel-example-generic" data-slide-to="1"></li>

          </ol>

          <div class="carousel-inner" role="listbox">

            <div class="item active">

              <img src="images/header-bg.jpg">

              <div class="carousel-caption"></div>

            </div>

            <div class="item active">

              <img src="images/header-bg.jpg">

              <div class="carousel-caption"></div>

            </div>

          </div>

          <a class="left carousel-control" href="#carousel-example-generic" role="button" data-slide="pre">

            <span class="glyphicon glyphicon-chevron-left"></span>

            <span class="sr-only">pre</span>

          </a>

          <a class="right carousel-control" href="#carousel-example-generic" role="button" data-slide="next">

            <span class="glyphicon glyphicon-chevron-right"></span>

            <span class="sr-only">next</span>

          </a>

        </div>