How do I build stronger friendships?

I’ve been participating in some hobbies and volunteering for several months now and feel like I have good discussions with people. I get their socials and numbers but when I invite them to hang out outside of hobbies and volunteering my messages either get ignored or they say they’re too busy and don’t offer to reschedule.

Is there anything I am doing wrong? How do I build deeper connections with people to hang out outside of my hobbies?