As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LAS - AIR TURNBACK - FLT 372 - DURING CLIMB CREW REPORTED THAT THE AIRCRAFT WOULD NOT HOLD CABIN CLIMB THRU 23000. CABIN AT 9900 AND CLIMBING, ECAM ACTION FOLLOWED. AIRCRAFT RETURNED TO LAS, DECLARED EMERGENCY, AND LANDED WITHOUT FURTHER EVENT. MAINTENANCE REMOVED AND REPLACED THE NR 1 PACK AIR COOLING BELLOWS IAW AIRBUS MM 51-52-00-200. PRESSURIZATION AND LEAK CHECKS WERE GOOD.