I posted something similar in the Amex 2-4-1 discussion yesterday. I had been looking to book Feb/March next year in F. All routes had loads of availability (apart from the usual suspects) then overnight just as I was about to book leave for work away they went.
I sent the BA twitter team a message today and got a reply saying how popular the rewards in F are and that they have likely sold out. I replied saying that the LHR-BOS route alone has over 60 days from Jan-Mar free alone, I find it hard to believe they ALL sold out....no reply yet. I'll keep you posted!