You're probably better off looking on the Aadvange site for the cheapest flights (milesaver which should match what BA will sell you) as it will give you a calendar of availability (and you want direct flights if you can because of BA's leg-by-leg pricing). http://www.aa.com/reservation/awardF...earchAccess.do

In fact aadvantage will only let you book 331 days in advance so the problem is probably that you're too early! (On ba.com I get an error rather than zero availability looking at the Christmas period, but no problems early December.)