A 32-year-old multiparous woman is evaluated for postpartum hemorrhage on postpartum day 3. She had a normal vaginal delivery 3 days ago with an estimated blood loss of 400 mL. Now she presents with heavy vaginal bleeding, passage of clots, and a tender, enlarged uterus. Hemoglobin has dropped from 11 g/dL to 8.5 g/dL. On ultrasound, there is echogenic material within the uterine cavity. What is the most common cause of late postpartum hemorrhage in this case?