I believe that BA's revenue management system has imposed a policy of selling seats before making them readily available to those who want to redeem Avios. Although my redemptions tend to be UuA from WT+ to CW rather than pure award redemptions in F, I think the same principle applies. There is much less availability on most routes over the course of the year, when BA can hold out and see if customers will purchase tickets, than there was several years ago, with more seats released for Avios redemption within a week to days before the date of departure, when BA may realize that its better to get Avios off the books rather than hope for someone to buy an unsold seat at the last minute.