I had the exact same issue. Tomorrow’s UA2042 was booked full in F for a while and fortunately one seat opened up. I checked my account for a buy up and didn’t see one ( this issue has been now happening for weeks). Gave up and called the 1k line, they quoted me $2k. I laughed and hung up. Was waiting for an offer at check in. But someone had already taken an offer (presumably). Is there a way to call and ask for buy up offers? Can web support help? Seems like this is a genuine website issue.