Second root canal on the same molar and there is still throbbing, pain, and extreme sensitivity to the tooth. Why?

May - First root canal after persistent pain and throbbing - xray revealed a shadow on the inside root. June - porcelain crown received, August - revisited the dentist who checked and adjusted my bite and prescribed 600 mg ibuprofin for the continued pain and throbbing. October - went back to the endodontist who sent me for a tornagraphy image. November- second root canal through the crown. pain and throbbing continues and is worse than before. Taking advil and tylenol for pain.