A321
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance
Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Aircraft; Human Factors
On climb out thru 19;000 feet we noticed the cabin altitude had gone above 9000 ft.  We notified ATC and leveled off at 20;000 ft.  ATC could not get us lower right away so I [exercised PIC authority] and we were given clearance to 14;000 ft and we started back to [departure airport].  The cabin altitude reached a max of 10;400 ft and the masks did not deploy. We returned for a normal approach and landing. After gate arrival and maintenance checking the airplane I was advised that a cargo door seal was installed incorrectly.
A-321 Captain reported returning to departure airport after experiencing loss of cabin pressure that was later traced to an improperly installed cargo door seal.