Well, you've run into the major problem with trying to have an all-inclusive sort method for RSS feeds. Look at the pubDate in the source, and you'll see what you appear to be doing is taking the first chars of the pubDate to use as a sort key, but I don't really see you calling a sort routine.
<pubDate>Sat, 31 Mar 2007 21:03:48 -0700</pubDate>
So, given that info, you would need to look at more of the date field to properly sort it. Also, you say this is working for other feeds, and I'd like to see the URL for ones that are working, as I suspect the date format is different. It's hard to know without having them to look at, though.
One, thing to try is parsing is testing the date format, then applying one of a few formatting translations to a sortable ascii format, hoping that you don't have language issues to blow up your algorithm. Sorting by date is not an easy task, but if you google using xsl and sort date you'll see many examples to try.