What is the hardest thing about building a community?

I’ve been thinking a lot about community building these days and going into a rabbit hole about what makes a successful community. I’ve asked this question in other forums and have received great responses. But it’s always a good idea to get different perspectives.

Basically: What is the hardest thing about building a community?

I want to learn from real-world examples of the challenges you’ve faced building your community? Also share your thoughts, tips and hacks that worked.

Thank you in advance! Look forward to hearing from all of you.