I don't think you're necessarily doing anything wrong, Jan. I never promote any of my stories on social media and I don't even have 1000 followers. However, I have had stories I wrote on "universal" themes go viral (e.g., being told I'm not pretty but still attractive, my mom trying to persuade me be more traditionally feminine and deferential).
I truly think it's a matter of luck to write on the right topic and then get your story in front of the right readers. The Medium algorithm is unpredictable!