What could cause for a page to get not indexed?

[nick23]

If a page is not indexed then what are the reasons behind it? Can anybody explain it to me which are the causes??

[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.

[lordp]

As stated earlier it could be because of many reasons. First you have to check that whether the page is crawl-able or not. That it is not blocked via robots.txt or .htaccess files. Then you have to check the navigation of the page from homepage and to homepage. Later then comes the part why search engines isn't indexing it.

It can also be of many reasons such as the page hasn't got much of a value, the content on the page is somehow inflicting with the search engine policies, absolutely ridiculous interface etc.