Enclosure tag present in Views auto-preview but missing in resultant feed xml

Hi, hope someone can help me?

I have a feed built using views_rss, which outputs correctly in the auto-preview in the Views UI. An example Item from the preview is below:

<item>
<title>This is a test title</title>
<link>http://www.dummyurl.com/linktostory</link>
<description>This is the teaser extract</description>
<category>Special Interview</category>
<enclosure>http://www.dummyurl.com/sites/default/files/test.mp3</enclosure>
<pubDate>03.14.13</pubDate>
<source url="http://www.dummyurl.com/admin/build/views/nojs/preview/home_subscriber_latest/feed_2">Latest for Subscribers</source>
</item>

However, if I save the view and browse (using Firefox) to the feed's URL, viewing the source shows the following - enclosure element is missing:

<item>
<title>This is a test title</title>
<link>http://www.dummyurl.com/linktostory</link>
<description>This is the teaser extract</description>
<category>Special Interview</category>
<pubDate>03.14.13</pubDate>
<source url="http://www.dummyurl.com/admin/build/views/nojs/preview/home_subscriber_latest/feed_2">Latest for Subscribers</source>
</item>

I just can't understand why this is happening! Any ideas?

Many thanks

JB