It takes a lot of observation and data collection as well. Due to competition, SEO takes a while to become higher up in the rankings. It can also be difficult to catch up with competition because of the fact they have been around for longer. If a website is continuing to work at SEO, they are still many steps ahead.
For More Information Please Refer: https://dynomapper.com/blog/21-sitemaps-and-seo/471-why-does-seo-take-so-long-for-search-engine-success