I came across an article yesterday that was an "ah ha" moment for me. About three months after my surgery (2007), I started getting a new pain, actually worse than what I had prior to surgery. This article states that they (doctors) feel that between 3-6 months post-op, problems can arise from inflammatory healing. Essentially, the doctors feel that fibrotic tissue develops in the epidural space around decompressed nerves.
If you have FBSS, I'd recommend you read the article. It is "Failed Back Surgery Syndrome: Pain that persists after surgery in a subset of patients". It's authored by Marc Russo. It's a good primer for things to come for those that have it.