Bone graft failure in L5-S1 fusion of 14 yo with spondylolysis

My 14 yo daughter is preparing for her second fusion surgery for spondylolysis.  She had bilateral fractures of her L5 pars interarticularis.  We tried to brace her with a Norton-Brown brace for 4 months.  At the end of that time, her slippage had gotten worse.  She was having more and more incidences of severe pain and sciatic pain.  All appeared well, after her fusion.  After extensive physical therapy, she was able to return to moderate physical activity (11 mos post-op)  When she went to her spine surgeon for a one year follow up, he was not happy with the results of the fusion.  There was no evidence of any of the bone graft that he had completely covered the L5-S1 area with.  Furthermore, you could still see the fracture line on one side.  It was still being held in place by the hardware, just no bone growth (or overgrowth).  She has had increased episodes of sciatic pain.  The first fusion was posterior.  Now the surgeon wants to do an anterior revision.  Is there any reason why in a young, mostly otherwise healthy teenager, that the bone graft would have failed?  Her blood work is all normal.  Should we do anything to help facilitate healing of the bone this time? Her only other medical diagnosis is mild eosinophilic esophogitis.