As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHL - FLT 1926 - UNABLE TO PRESSURIZE ACFT ON CLIMB. FLIGHT RETURNED TO PHL WHERE LANDING WAS WITHOUT INCIDENT. MAINTENACNE FOUND RT PACK NOT ABLE TO SUPPLY SUFFICIENT AIRFLOW. RELEASED ON MEL 21-1. REPLACED AIR CONDITIONING ACCESSORY UNIT M324. OPS CHECK NORMAL. CLEARED MEL. (W)