It simply means that there were no seats available at the time you called. Perhaps someone got the seats ahead of you, but I doubt it. I don't think that they ever released seats into reward inventory on those flights nor will they.

jetBlue does NOT always make seats available on every flight. For high demand dates (and a transcon on Thanksgiving Sunday is certainly high demand) it is possible that seats will never be released for reward inventory. It is possible that they will open up later, but I highly doubt it.