As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FTL NR 6586 - IAD-ISP - DURING CLIMB-OUT, RETURNED TO IAD DUE TO THE AIRCRAFT NOT PRESSURIZING. AIRCRAFT LANDED WITH NO FURTHER INCIDENTS. MAINTENANCE REPLACED LEFT P3 LINE AND RECONNECTED RIGHT ACM DUCT. AIRCRAFT WAS RELEASED AND APPROVED FOR RETURN TO SERVICE. REFERENCE VTZA96129.