Every website that hopes to place among the top positions in generated results pages on Google spends a great deal of effort deciding on memorable and distinct terms and phrases to associate the site and its contents with. When Google's crawlers discover a website, the search engine's algorithms attempt to pick up on the presence of such terms within each of its pages and make an educated guess as to whether that page is an authoritative resource for those terms.
Being able to base a site's message on a keyword that is relevant to its company's industry and yet is not already heavily featured between the site's competitors in that industry is an art form because striking such a delicate balance requires a lot of research effort and expense. Even in the best of times, the competition might be fierce enough that a webmaster who seemingly does everything right might still not be awarded the highest possible SERP positions for keyword niches by Google's persistently evolving and concealed standards.
One potential obstacle that most webmasters would be forgiven for never expecting, however, is when Google's service itself suffers a technical difficulty that has no connection to the SEO efforts of websites trying to get themselves noticed on it. Early in April 2019, many websites that were fully indexed had sizable fractions of their pages no longer appear anywhere near their original positions in Google's SERPs because those pages had accidentally been taken off Google's overall indexing database. Many websites reported that their positions were either moved to far later pages in the SERPs or taken off the results of those SERPs entirely.
Since this is an issue of Google's own making, web owners can expect to have their websites indexed again and their missing pages restored over time. However, Google Search Console offers a service in its "URL Inspection" menu that allows owners to input the URLs that are believed to have been affected. If Google considers those pages not to have been indexed because of the glitch, it will allow the "Request Indexing" option to be used. For more information click here https://www.reddit.com/r/SEO/comments/bb6fgx/whysomekeywordsisnotshowingon_google/.