I had booked HKG-BOS Jx2 which we flew in February. At T-11 days with F4 one opened up and I upgraded one into F. Watched until T-6 hours for another to open up. One never did and we departed with 3 empty F seats.
A busier flight is better than a light one. If J is full they are more likely to open upgrades to F as it is easier to sell a last minute J. If J is 1/2 full as it was on my flight, from a cost perspective, it makes no sense to offer upgrades to F and provide Pajamas, free flowing premium Champaign and Caviar when there is no offsetting revenue potential to cover those costs. It is cheaper to leave the F seats empty.
James.