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By giving me that link you indirectly answered my question so thanks! A Sitemap file located at http://example.com/catalog/sitemap.xml can include any URLs starting with http://example.com/catalog/ but can not include URLs starting with http://example.com/images/. Having all my sitemaps in a /sitemaps/ folder meant that each of those sitemaps could only include URLs beginning with example.com/sitemaps/. I moved all my sub-sitemaps to my root (similar to how wordpress does it) so that they cover the scope of my entire site.