What could cause for a page to get not indexed?

Below a reason for a page not indexed

1. Have you checked your robots?
2. Are you pointing the Googlebot to a redirect chain?
3. Have you implemented the canonical link correctly?
4. Maybe you have exceeded your Crawl budget
5. Is your page actually an orphan?

The first thing to check robots.txt file & tag. if they exclude the page or not.

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