WordPress natively only refreshes RSS Feeds every 12 hours. It caches the results for this periode, so if you attempt to retrieve new RSS information, it’s likely WordPress will see if its cache hasn’t expired yet. Also, see the API Reference for more information.
The API Reference can be a little unclear about the use of this filter. To override the interval in which WordPress refreshes its feed cache, put this code in your functions.php file. The code below will set the interval to 100 seconds.
add_filter('wp_feed_cache_transient_lifetime', 'my_rss_interval');
function my_rss_interval( $seconds ) {
return 100;
}
On a side note, I’m not sure what good it will do to set the interval so low. I don’t know what you are trying to achieve with this, but there are performance reasons why the feed is cached. Perhaps if you explained what you are trying to achieve doing this, there is a better solution than hammering another site’s feed.