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The biggest issues are ill-formed feeds, charset issues, MIME type issues and server misconfigurations (which often cause the previous two problems). Your feed should be declaring its charset via a HTTP header, and have the MIME type
It could be because the rss page is a php page. I have a php script that updates with the latest news via my database. It puts it in an rss format for others to view.
I have it working fine on my website on the homepage. I have an rss reader script that shows the latest 5 rss feeds from my feed.
Hope that made sense ;)