John Clark alerted me that Copia has been missing from Planet XML. I noticed it was also missing from PlanetPython. The Planet XML problem turned out to be because the XSLT used to convert the Atom feed to RSS 1.0 was written for Atom 0.3, and so stopped working when I upgraded Copia to Atom 1.0 late last year. I updated the XSLT and that's sorted out (as an unintended result I pwned that planet for a day or two). Planet Python uses a category feed in Atom from Copia, and I think the problem is that the version of Planet used in this aggregator does not yet support Atom 1.0. Planet XML uses its own aggregation software and has supported Atom 1.0 for a while.
There have been moves to update (see this message, for example). Now that FeedParser 4.0 is out with Atom 1.0 support, I expect most planets will start to correct their Atom deficiencies.
Meanwhile, John and I have been working with Sylvain Hellegouarch on yet another planet, using our own aggregation software. More on that later.