I was getting some Google Webmaster errors because I had some empty children on my sitemap.
After excluding the relevant taxonomies, my sitemap would still not update.
I enabled and disabled the sitemap, but this did not work. In fact, the old sitemap still remained even when disabled.
Clearing any caches did not seem to do anything.
Turning off Cloudflare did nothing.
Checking my nginx rewrite rules did nothing.
What seemed to work was updating a post. This seemed to triggered the update and I had a new sitemap.