I spoke with the doctor this morning and she was very positive about Nathan's recovery. She told me the disk that ruptured this time had been damaged for quite some time. So, its no wonder Nathan's recovery was stalled. Also, she said the previously operated disk had a lot of scar tissue that needed to be cleaned up.
Lesson: Do not let anyone other than a neurosurgeon perform this procedure on your dog, even if it means driving a distance to have it done properly.
I just checked in with the hospital. Nathan is already standing again. Hard to believe just a few hours after surgery he is better than he was yesterday. I'm already feeling hopeful that he will be even better than he had been over the last months.