What was found
CLT - FLT 521 - AIRCRAFT WOULD NOT PRESSURIZE AFTER TAKEOFF. STOPPED AT 8,000 FEET ABOVE AGL, THEN RETURNED TO CLT. ON DESCENT AT 2,000 FEET, AIRCRAFT STARTED TO PRESSURIZE. MAINTENANCE REPLACED OUTFLOW VALVE DUE TO STICKING. OPS CHECKED OK. (X)