All RSS feeds are in XML format but not all of them are of text/xml content type which of course they should be, since they are xml. Aggreg8 does not work for Feeds with content type not set o text/xml, this in my opinion is not a flaw but an oversight on the part of those hosting the RSS feeds. Below you can see how to set the content type of a file using various languages :

PHP :
<? header("Content-type: text/xml); ?>

ASP:
<% Response.ContentType = "text/xml" %>

JSP :
<%@ content_type="text/xml" %>

If everybody did this then Aggreg8 would work for all feeds… it would make me a lot happier and my inbox wouldn't be so full.