Bootstrap links become unclickable on small screen

I am using bootstrap to design a responsive website. The links work fine when the browser window is maximized.But when i test the same on a mobile window the links become unclickable.

<div class="container-fluid">
<div class="row" style="text-align:center">

<div class="col-lg-9 pull-right col-md-9 col-sm-9 col-xs-4 pull-right" style="height:150px">
<span id="lines" onclick="openNav()"> </span>
</div>

<div class="col-lg-3 col-md-3 col-sm-3 col-xs-8 content_side">
   <div class="col-lg-10 col-md-10 col-sm-10">
   <img src="images/logo.png"  class="img-responsive" id="img_logo" alt="logo"/>
   </div>

    <div class="col-lg-10 col-md-4 col-sm-4">
    <ul>
    <li><a href="pages/a.html"> about </a></li>
    <li><a href="pages/b.html"> contact </a></li>               
    </ul>
    </div>
</div>

</div>
</div>