More and more, first class sleeping accommodations seem to sell out very early. Since the fares are cheaper the further out you go, many who like to travel Amtrak, will book their rooms far in advance, especially retired seniors. I think if you booked the "special" coach fare and tried to upgrade on board, you could. If you tried to book the special fare and then combine it with a sleeper days in advance, I could see how that would make it ineligable. If there are open sleepers, they try to sell them and sometimes even make announcements on certain trains.