Have managed to redeem 5 in F on the same flights to and from South Africa in Sept 2010 for travel in Jan 2011. Coming back we were the only ones in F and BA opened up the other 9 seats in F (eg whole F cabin). As another poster said if you are looking for narrow date ranges in April/May where lots of holidays in the UK fall, it is almost certainly a case of availability having been snapped up by others.

Given how much you spend on BA are you not close or at Gold Guest List? That allows you to convert revenue seats to award seats.