As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, THE CREW REPORTED THAT THE LANDING GEAR WOULD NOT RETRACT. THE FLIGHT RETURNED TO IAD AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE PERSONNEL RESEATED THE UNDERCARRIAGE RELEASE VALVE. OPERATIONAL CHECKS WERE SATISFACTORY AND THE AIRCRAFT WAS RETURNED TO SERVICE.