I have an ad and a link in a page like so:

```
<!DOCTYPE html>
<head profile="http://www.w3.org/1999/xhtml/vocab">
<title>DPT - Asynchronous + Single Rest Architecture</title>
  <script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"></script>
  <script type='text/javascript'>
    var googletag = googletag || {};
    googletag.cmd = googletag.cmd || [];
    (function() {
    var gads = document.createElement('script');
    gads.async = true;
    gads.type = 'text/javascript';
    var useSSL = 'https:' == document.location.protocol;
    gads.src = (useSSL ? 'https:' : 'http:') +
    '//www.googletagservices.com/tag/js/gpt.js';
    var node = document.getElementsByTagName('script')[0];
    node.parentNode.insertBefore(gads, node);
    })();
  </script>
  <script type='text/javascript'>
    googletag.cmd.push(function() {
    googletag.pubads().enableAsyncRendering();
    googletag.defineSlot('/1001256/Home_Top_Leaderboard_728x90', [728, 90], 'div-gpt-ad-1342320102476-72').addService(googletag.pubads());
    googletag.pubads().enableSingleRequest();
    googletag.enableServices();
    });
</script>
</head>
<body>
    <div id="div-gpt-ad-1342320102476-72" style="width:728px; height:90px;">
      <script type="text/javascript">
        googletag.cmd.push(function() { googletag.display('div-gpt-ad-1342320102476-72'); });
      </script>
    </div>
    <a id="refresh" href="#">Refresh ad</a>
    <script type='text/javascript'>
    (function ($) {
        $('#refresh').click(function() {
         googletag.cmd.push(googletag.pubads().refresh());
        return false;
        });
    })(jQuery);
    </script>
</body>
</html> 

```

The first time I click the link, the ad refreshes. All other times nothing happens. Even just calling the refresh() method in Firebug etc does nothing after the first time. 

What's wrong with the above?