I have had this occur for a couple of different reasons. Sometimes .com (commonly referred to as .bomb) just doesn't work with you. A call sometimes provides more accurate information. It often has a mind of its own.

And others have it can be too early. One of the things I do, is look at the seatv map. For example looking at your departure date, put if as if you are buying business class seats, if there is a lot of wide open spaces; it often means UA is not yet ready to provide upgrades on that flight. If a lot of the seats are already booked, that's a different clue.

I suspect that eight months out, very few seats have been sold.