Boostrap 3品牌标识隐藏在导航栏中

Question

这个简单的事情让我感到沮丧几个小时。我有一个简单的导航栏，有一个品牌标志。但是徽标有时会从导航栏中消失（仍然加载但隐藏在下方）。

HTML：

<nav class="header_blue navbar navbar-default navbar-fixed-top" role="navigation">
    <div class="container">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle pull-left" data-toggle="collapse" data-target=".navbar-ex1-collapse">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>        
          <div class="navbar-inner">
           <a class="navbar-brand" href="">
                <img id="my_img" style="display:block !important;" src="img/logo_img.png" alt="logo_img" class="img-responsive">
           </a>
          </div>        
        </div>
        <div class="collapse navbar-collapse navbar-ex1-collapse">
            <ul class="nav navbar-nav">
            </ul>
             <div class="nav navbar-nav navbar-right" ng-hide="currentUserSignedIn">
                 <ul class="list-inline pull-right">
                    <li>
                        <a href="" id="signup_btn">Register</a> 
                    </li>

                     <li>
                        <a href="" id="sign_in" data-target="login_modal" >Sign In</a> 
                    </li>
                 </ul>
            </div>
        </div>
    </div></nav>

CSS：

.header_blue{
background:none repeat scroll 0% 0% #073648;}