I'm not anything resembling an authority on this, as I am yet to even launch my coworking space, but I take a different tack on this than Hillman does.
I don't see what you're describing as any sort of failure. You're offering space, people are using it toward a productive end - in the way they want to. Your job doesn't have to be to push them to spend their time in a particular way, in this case more socially. You create the opportunity for them to interact with each other, but if they prefer to go into their office and get work done without socializing, what is that, in the words of Hillman, a symptom of a "bigger problem"?
Is your space there to be a resource for people, or are you there to tell people how they should be running their business, using their time, etc? Is it more important to you that people interact in the way you think they should, or that you're providing a valuable service? If they don't need help, and just want to get work done in private... I'm failing to see anything at all wrong with that, or the opposite... or somewhere in between.
SK