Check what your sitemap really declares
Sitemaps declared in robots.txt are followed first, the way the spec says to — including nested sitemap indexes most checkers stop at.
One file is rarely the whole sitemap
robots.txt comes first
The spec says crawlers look for a Sitemap: line in robots.txt before anything else. A sitemap at a non-standard path with no declaration is a sitemap most crawlers never find.
Indexes get followed
Large sites split their sitemap into an index pointing at dozens of child files. Checkers that stop at the index report a handful of URLs for a site with thousands.
Broken children stay hidden
A sitemap index can list a child file that 404s or contains malformed XML. Nothing surfaces that in Search Console until indexing quietly falls behind.
What this doesn’t tell you: This reads what your sitemap declares, not what Google indexed — a URL listed here can still be excluded, which only Search Console can confirm.
Run all of this on your whole site, continuously
These tools check one thing once. SEO Engine watches every page, every week, and writes the fixes for you.