This happened a long time ago, about the same time or just before BA announced their guaranteed release of x seats in lower cabins.
No F is guaranteed to be released for plebs like us. It never was before, but it was often available.
Partner airlines are also now more heavily restricting F reward availability for their customers and even more so for partners (part from possibly QR) because scalpers were booking them all up and selling them on.
Your best bet is to wait until less than a month to go and hope to find one then when rev management allow it.
But even that is not always possible and it varies a lot based on route as you have seen. They will be snapped up when released, so you probably need to have a premium membership of an alert service and be on the ball.