My Granddaughter Refuses To Have A Bowel Movement. She Holds

my granddaughter refuses to have a bowel movement. she holds it and says it hurts to go. she has always had a constipation problem since birth. her doctor has given my daughter a prescription for Metamucil and they give her prune juice ang fiber but she sstill wont go on the toilet. she will go in her pants but complains about the way it hurts. could there be another problem they cant see