This has happened to me twice, lately, on the same reservation, for late October. Granted, it was 15 A&B, SFO to SGN, but no one at UA 1K can explain why it happened. Yes, I know all about FAMs getting those seats, but....
The first time it happened I called and a great agent in the Honolulu service center ended up moving us to a flight a day earlier, when 15 A&B were available. This, after almost an hour of trying to figure out why we'd lost the seats in the first place. 4 days later I checked the itinerary and found that we once again did not have seats assigned. I did have a red note about a schedule change, at ual.com. This time I pretty much went directly to a supervisor in HNL. She could not find ANY change in our itinerary, except for 15 A&B being taken away - not a different class of plane, not even a 1 minute change in the flight/time schedule. Despite her best efforts she could not find out who was assigned to the seats - she went through every passenger who had a Business Class ticket, and none were assigned 15 A&B, but she could not move us back into the seats. Very frustrating!! After discussing it all with her, I asked to have our dates changed back to our original start of trip date, and we'll be in 13 G&H. I made these reservations, and upgraded them, in March.
Some good did come of this, as she was able to tell me the dates that the next 3 schedule chages would be loaded - August 3, September 6 and October 4.
Kate