I'm so sorry. As I read your post, the thought came to mind that Methotrexate, which you had been considering, would probably stop new bone from growing. I know that you don't take Methotrexate, but is it possible that you were taking other any other drugs that interfered with cell division and prevented new bone from forming?

My second thought is that you might have been too active postoperatively, so excess movement and torque prevented healing. If that is the case, a second surgery would have a better chance of being successful if you reduce the stress on the spine for a while.

I'm just throwing out brainstorming ideas hoping that something plausible and hopeful comes to you.