Google's Indexing API Is Not Guaranteed to Be indexed

John Mueller, an advocate for Google Search, said that content is not guaranteed to be indexed in response to a site owner asking why their content wasn't appearing on Google. Mueller explained that the Indexing API is only meant for specific types of content, such as livestreams and job postings. He also went on to say that Google considers many factors when determine if content is worth indexing. A Reddit user asked another Reddit user for ways to make their website more valuable. Mueller responded saying that it can be difficult to make sure that content published by other people meets Google's standards. Mueller went on to say that the more you can do to make sure that the indexable content on your website is easily found and valuable to the web, the more likely Google is to pick it up quickly. A Reddit user suggested that websites could signal to Google that there is new content available by linking to it from the home page. He said that there is no easy solution to that issue, such as using Google's Indexing API.

(source: https://www.searchenginejournal.com/googles-john-mueller-search-is-never-guaranteed/470388/)