What was found
CREW REPORTED AIRCRAFT NEVER PRESSURIZED DURING CLIMBOUT. DIFFERENTIAL PRESSURE CLIMBED WITH AIRCRAFT. CABIN ALTITUDE CAUTION MESSAGE APPEARED, FOLLOWED BY MASTER WARNING CABIN PRESSURE. CREW RETURNED TO FIELD AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE INSPECTED AIRCRAFT AND FOUND AFT CARGO DOOR UNSEATED. MAINTENANCE CLOSED AFT CARGO DOOR AND PERFORMED OPERATIONAL AND PRESSURIZATION CHECKS USING AIRCRAFT MAINTENANCE MANUAL. AIRCRAFT RELEASED FOR SERVICE.