On Twitter, Google’s John Mueller confirmed that Google does not index all pages on the web. He suggested to improve the internal links on a website.
One thing to add here – we don't index all URLs on the web, so even once it's reprocessed here, it would be normal that not every URL on every site is indexed. Awesome sites with minimal duplication help us recognize the value of indexing more of your pages.— ? John ? (@JohnMu) April 7, 2019
That issue is resolved in the meantime, though that doesn't mean we'll index all pages. I'd use the submit-tool for important pages, and find ways to better link them internally within the site.— ? John ? (@JohnMu) April 12, 2019
You can check the internal link structure of your website with the website audit tool in SEOprofiler.