Not indexed does not always mean an error. Intentional noindex rules or canonical selection can explain exclusions. First establish whether the page should appear in search.
Inspect reasons and example URLs, then use URL Inspection for important pages. Sitemap filters and trends help, but report counts do not recreate the complete live state of a site.
When counts fall, check releases, status codes, redirects, noindex and canonicals. Pagination alone does not make pages duplicates or automatic exclusion candidates; inspect their content and purpose.