Weird Indexing Issues with the Pages and Rankings

[mrmrsteven]

When I found the my page was non-existent on the search results page, I requested Google to index my page via the Search Console. And then just a few minutes after I did that, that page rose to top 3 ranking on the search page (with the same keyword and browser search). It happens to most of the pages on my website.

Maybe a week later the rankings sank again, and I had to do the process again to make my pages to the top. Any reasons to explain this phenomenon, and how I can fix this issue?

Thank you in advance.