bootstrap 的media-bottom为何无效

 <div class="media-body">
<h4 class="media-heading">打令</h4>
<h5 class="media-heading">夜视单反</h5>
<div class="media-bottom">
<span>人气指数</span>
<div class="progress progress-striped" >
  <div class="progress-bar progress-bar-success" role="progressbar" aria-valuenow="40" aria-valuemin="0" aria-valuemax="100" style="width: 40%">
    <span class="sr-only">40% Complete (success)</span>
  </div>
</div>
</div>
</div>

我写了media-bottom为何无效呢，元素没有去到底部

打令

夜视单反

人气指数

40% Complete (success)

 <div class="media-body">
<h4 class="media-heading">打令</h4>
<h5 class="media-heading">夜视单反</h5>
<div class="media-bottom">
<span>人气指数</span>
<div class="progress progress-striped" >
  <div class="progress-bar progress-bar-success" role="progressbar" aria-valuenow="40" aria-valuemin="0" aria-valuemax="100" style="width: 40%">
    <span class="sr-only">40% Complete (success)</span>
  </div>
</div>
</div>
</div>