When on mobile or when I resize my browser the navbar dropdown menu is not dropping down when clicked. I have put the navbar code here for elaborating my issue. Thanks in advance for your help! 

```
<nav class="navbar navbar-default navbar-fixed-top">
  <div class="container-fluid">
    <div class="navbar-header">
      <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="myNavbar">
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
    </div>
    <div class="collapse navbar-collapse" id="myNavbar">
    <ul class="nav navbar-nav navbar-right">
      <li><a href="#home">Home</a></li>
      <li><a href="#about">About</a></li>
      <li><a href="#portfolio">Portfolio</a></li>
      <li><a href="#contact">Contact</a></li>
    </ul>
  </div>
 </div>
</nav>

```