It’s quite easy to spot what’s going wrong.
If the people you invite don’t visit the website, you’re invitation isn’t good enough.
If the people visit but don’t sign up, the interface isn’t right.
If people sign up but don’t stick around, you’re not properly welcoming them into the community.
If people become regular members but don’t invite others, you’re not motivating them to do so.
If people invite others but they don’t join, you’re not giving them the tools and information they need to do it.
If people don’t visit the website as much as they used to, you’re not fostering the relationships and involvement they need.
It’s very easy to spot what’s broken. So fix the problems quickly.