I guess its "Quota" - meanwhile (and that indeed has changed over the last few weeks) TG permits P- and J-Class bookings up to the very last seat, while the subclasses for award tickets and upgrades seem to have been reduced and further limited.
And as per their latest news release, TG seems to have a reason for it as they claim they are doing very well with "excellent forward bookings for July"..
But you should try again very few days before departure .. it might work out!