Symfony 2 data not rendering in RSS.XML file, but the data is there in the markup code

I am creating an RSS feed for my project, and currently the file that holds the RSS data (the XML file) is like this:

<?xml version="1.0" encoding="UTF-8" ?>
<rss version="2.0">

<channel>
    <title>RSS Example</title>
    <description>This is an example of an RSS feed</description>
    <link>http://www.domain.com/link.htm</link>
    <lastBuildDate>Mon, 28 Aug 2006 11:12:5开发者_C百科5 -0400 </lastBuildDate>
    <pubDate>Tue, 29 Aug 2006 09:00:00 -0400</pubDate>
    {% for latestnews in news %}
    <item> 
        <title>{{ latestnews.title }}</title>  
        <description>{{ latestnews.content | truncate(500) | raw }}</description>  
        <link>../app_dev.php{{latestnews.url }}</link>  
        <guid isPermaLink="false">{{latestnews.url }}</guid>
        <pubDate>{{ latestnews.lastedit | date('jS F Y') }}</pubDate>  
        <media:thumbnail width="144" height="81" url="{{ latestnews.mainImage }}"/> 
    </item>  
    {% endfor %}
 </channel> 
</rss> 

However, when the page is viewed in the browser, everything except for the data and tags contained within the {% for latestnews in news %} and {% endfor %} displays. The actual latest news articles don't display. When I check the mark up code however, the latest news articles are displayed.

I've looked at other RSS feeds (such as the BBC News one) and I can't see any difference in what I've done to what they've done. Do I have to add something, a specific Symfony code, that makes it render items when it's inside a loop?

Cheers

The problem stemmed from using the <media:thumbnail> tag. Once this was removed all of the articles displayed properly. Bizarrely, it was Internet Explorer that helped me solve it!