Here comes a lot of factor. What kind of site is it? What kind of links does it have? And coming to the content, does google and other search engines really find that content usable to its users.
There can be many reason .such as Sitemap submission ,crawling ,listing ,content ,backlink,page-speed,on-page,off-page optimization and PA - DA for competitor.
First of all we should find out what is the issue which is hurdling website indexing. It can be fro the following: 1. Robot.txt XML file is Mistakenly excluding that page or folder. 2. URl is containing the excluded parameters. 3. Duplicate content in two or more pages 4. Not enough backlinks for crawler to take deep interest in website or page. 5. There is no link present on indexed page for your page. According to above problems we can use the solutions.
continuing with the above points... 6. Low page load time 7. Page with meta "noindex & nofollow" 8. You have a crawl error 9. Your website's privacy setting (If you are using Wordpress site) 10. You have Ajax / Javascript issue (It is found in some website) 11. Hosting downtime 12. Might be you get deindexed. I hope this will help.
And I think the unmentioned is, sometimes Google is kind of a douche. Believe it or not, they definitely manipulate things to make more money above everything else, which does make sense looking at it from their perspective. For example, the other day I got a tumblr decently ranked and was on page 1 in rank 1-3 for every search engine but Google. With Google, I didn't even exist in 100 results for the exact match on the subdomain. I double checked, and most certainly did submit all the sitemaps within 20 minutes of each other Bing/Yandex/Google. Well Not beast mode doing ok for 6 days cpl crappy tumblr shares. Doin it! So ya... Sometimes Google's just on that slow roll drag their feet not cooperate kinda stuff
Go into search console and ask Google to index the pages? That would be the first thing I do. I came across this post as I am researching if there is anything else I can do to fix my situation. I don't know what happened but some how I accidentally (or one of my 3 kids under five who are chronic button pressers) clicked "NO" in the Allow search engines to show this Category in search results in the Yoast SEO plug in. I found out today when in google search console, that my page was removed from the index!! Freaking out. I was ranking quite well before this, I am worried that the page may never recover. I am pretty sure it has been less than 24 hours as I looked at this particular page yesterday. I obviously have changed it to YES, and told search console. Anything else to do to? I can't work out how to start a new thread. lol