Something similar, or the same just happened to me too. Still shows under my reservations but when clicked it goes to an error page with:
! We were unable to review the latest information for this itinerary.
I haven't called yet, it's viewable and looks fine on checkmytrip.
Booked through BMI in February. Might have something to do with as they ended their agreement with UA just about the time I could no longer see it. Involves only UA flights. Shows up fine in BMI's system too. UA iphone app says the confirmation code is wrong, tried all the other confirmation codes that UA had for the reservation and none worked.