What was found
MAIN DOOR VENT DOOR: UPON TAKEOFF IT WAS NOTED THAT AIRCRAFT WOULD NOT PRESSURIZE. WE TRIED MANUAL MODE PER QRH. DID NOT HELP. STAYED AT 10000' MSL AND FOR DIVERSION AIRPORT. AFTER 45 MINUTES PRESSURIZATION STRTED WORKING AND WE SUSPECT VENT DOOR NOT CLOSING WITH HANDLE IN DOOR./// CORRECITVE ACTION/// POWERED UP APU, STARTED L/H AND R/H ENGINES. PERFORMED PRESSURIZATION CHECK GOOD. NO DEFECTS NOTED.