Here's a great website that gives writers some basic info on how to market themselves as writers. If you are simply putting your books up on Amazon and forgetting about them, everyone else will too! (And yes, I'm guilty of that, too!)
You need to make yourself known, get yourself out there, so that your books have a chance to grow in popularity.
Check out this website to learn how to start: https://becomeawritertoday.com/marketing-for-writers/