Google’s John Mueller said on Twitter that Google does not index all pages that are in a sitemap. He also said that it was impossible to index all pages on the web.
Which issue do you mean, "indexed, not in sitemap" or like Martin mentioned, the opposite? We really don't index all pages in sitemaps or on the web, it would be impossible to do so.— ? John ? (@JohnMu) November 27, 2019
Are your web pages indexable?
Use the website audit tool in SEOprofiler to check if your web pages are indexable: