What Causes Poor Union Of Bones On X-ray After Having A Cast For Fracture?

MY 13 YEAR OLD DAUGHTER HAD A SPIRAL FRACTURE OF THE TIB AND FIB.THE DOC OPTED TO CAST IT AND 11 WEEKS OUT IT LOOKS AND FEELS GREAT.SHE EVEN PUT SOME WEIGHT ON .SHE IS IN A FRACTURE BRACE.THE PROBLEM IS THE X-RAY DOES NOT SHOW MUCH HEALING.WHAT DO YOU THINK