PDXalways

Does anyone have any insight as to why UA flight 438... departing DEN at 11:30 a.m. and arriving IAD at 4:50 p.m... is not available to book at all in June? It's been driving me nuts; my only daytime west coast to east coast itinerary option relies on that flight, as it's the only one that allows me to leave the west coast in the morning, connect in DEN, then connect again in IAD.

UA 438 is published on the schedule every day, and it actually operates each day with plenty of passengers on board. But united.com never displays it as a booking option - in any class of service. It allows daily booking on the other two DEN-IAD flights, departing at 12:18 p.m. and 6:28 p.m., but those flights are too late in the day to allow for connections in IAD during its sole hub bank of flights (4:30-6:00 p.m.) in June. I've been checking on this nearly every day for about two weeks now... and it's always the same. No booking availability, on any day in June, for UA 438. Yet it's on the timetable throughout June - and has been operating daily. Very frustrating.

Any insight? Revenue management mistake by UA? Or is the flight literally full every day for the next 30 days? (I find that hard to believe...)

EDITED TO ADD: I finally found seats available on one day - this Thursday, June 4. But I can't see the flight being available on any other days.