Today we saw that our sitemaps don't working. Because a few months ago we switched our servers apache to nginx. And forgot to add rewrite rules for sitemaps. Uh!
We just fixed our sitemaps with adding to rewrite rule for nginx.
rewrite ^/(.*/)?sitemap.xml /wp-content/sitemap.php last;
Maybe this helps someone;
Note: WPMU DEV crew, can you add this info to simple sitemap's page. Because there is only apache rewrite rule.