If the idle current is good, the tubes probably are not flattened.
Flat tubes will behave the way 543 described. This is a frequent result of running the amplifier from a 120-Volt outlet without throttling down to about half power. Low line voltage reduces the temperature of the tube's filament. Keying it with that temperature too low can screw up the chemistry that allows full current from the cathode.
I would turn the Load control to the halfway point, key it and see if the Plate Tune control will show a peak that way. If it does, go back to the Load and set it for max modulated power, then back to the Plate Tune.
Lather, rinse repeat until both knobs are at a peak position.
Could be your Load control is just set totally wrong. It should never have to be turned that far unless there is another problem.
73