What could cause for a page to get not indexed?

[greeindseo2019]

If a page is not indexed some reason are :

Your Site is Indexed Under a www- or Non-www Domain.
Google Hasn't Found Your Site Yet.
The Site or Page(s) are Blocked With robots.txt.
You Don't Have a sitemap.xml.
You Have Crawl Errors.
You Have Lots of Duplicate Content.
You've Turned On Your Privacy Settings.
The Site is Blocked by .htaccess.