col里嵌套row,被嵌套的那部分栅格布局没有效果了，为什么啊？

<div class="container">

   <div class="row">

   <div class="col-sm-9"> 

<div class="jumbotron">

<h1>Hello, world!</h1>

<p>基本介绍</p>

</div>

    <div class="row">

      <div class="col-6">

         <h2> 标题 </h2>

         <p> fsdhfhsjkhskfhksaffsfjlsfjkdsjfklsdjflskfjlksjflksjflsjflsjdflkdsavhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhkjsahldfjkfdassssssssssssssssssssssssssssssssss  </p>

         <a class="btn btn-default"> 更多</a>

      </div>

    </div>

   </div>

   <div class="col-sm-3">

<ul class="list-group">

<li class="list-group-item">Cras justo odio</li>

<li class="list-group-item">Dapibus ac facilisis in</li>

<li class="list-group-item">Morbi leo risus</li>

<li class="list-group-item">Porta ac consectetur ac</li>

<li class="list-group-item">Vestibulum at eros</li>

</ul>

</div>

</div>

</div>