There is a setting for "Thread Cutoff" which by default refers to the thread creation date. The default setting is 30 days, and although you have recent posts, it looks like the newest thread on your forum was created several months ago, so that would be why nothing shows up in your RSS feed.
You can add lastpost=1 to the url to make it use the last post date instead of the thread creation date. So for example this url:
http://management.ind.in/forum/external.php?lastpost=1 has a feed with some content. If you can't add that to the url everywhere you need to, you could force it to be the default by creating a new plugin using hook external_start, and code like this:
PHP Code:
if (!isset($_GET['lastpost']))
{
$vbulletin->GPC['lastpost'] = 1;
}