Maybe it's time for a repeat investigation. You might have an unstable spondylolithesis or perhaps the pain is coming as a referral from the bony vertebrae themselves. The fact that the block gave immediate relief suggest the buttock referral is in fact a referred pain - as you say maybe not from the nerve itself but perhaps from surrounding structures e.g. joint/bone. Now you need someone expert is working with you on restoring that lumbosacral area.