You know, it's really less of what you're doing wrong and more of identifying what you're doing right, doing more of that, and learning new methods to try using to promote yourself.
Most of us experience a long uphill struggle in the beginning when trying to find our readers. Like what most have already said, it takes time to grow. And even then, a lot of artists notice plateaus in their growth where they'll sit at the same view/sub count for months on end.
During these periods is when it's good to try new methods so you can potentially reach new readers. Stuff to really think hard on is what's your message, an elevator pitch for your comic, your target audience, niche markets, age group, where to find these readers, how to be more engaging, what type of social media posts do they find interesting, and new communities to join.
A comic artist's job is never done. There is always something to do and new things to learn and try. TRIAL AND ERROR are key!