This is for an issue I was able to resolve, but I wanted to post here in case anybody else runs into the same thing or if this is a bug that the SF team isn't aware of.
We have a site running Sitefinity 10.0.6400.1 Professional Edition, and we've been running into issues with the Sitemap Generator.
When manually generating a sitemap, the generator worked without any errors, but the generated file was missing lots of pages, even though the pages were published and had the "include in sitemap" setting checked.
If I updated an existing page that's currently appearing in the sitemap, for instance by unchecking "include in sitemap" or changing the priority, the changes were showing up when I regenerated the sitemap, so I knew it was getting updated. I compared a "working" page with one that wasn't showing up in the sitemap, and everything was nearly identical except for the page content itself.
One final thing I noticed is that newly created pages were showing up fine, it's just pages that were originally created a few months ago that weren't showing up. Finally, I tried unpublishing / republishing some of the affected pages, which did the trick. Just clicking Publish without first unpublishing didn't seem to have any effect.
Not sure why that was happening - does anybody know if this is expected behavior? The older pages were initially migrated over from a staging server, but it was done through a full database backup/restore, and like I said, I didn't see any differences between a working and non-working page. Anybody else run into this?