This seems to happen often - flights that don’t arrive often here won’t be sent at all via the FlightXML method, but do show up on the arrivals page on the FlightAware site.
Do they appear on the airport enroute boards while they’re in flight or just the airport arrivals boards after arrival? The Arrived FlightXML call includes them.
Watching manually on Flightaware.com, they do show up on the enroute board (which is how I know they are public flights and not blocked). The Enroute method doesn’t include them. My application doesn’t care about arrived, only enroute. In an unrelated issue, enroute is now also returning flights that are not yet enroute but which are planned and without an actual departure time.