Maybe you have the horizontal output tube plate connected to a wrong terminal, in that the output pulses are going thru too much of the flyback coil. That would lead to lower high voltage and lower boost...
Check with an ohmmeter (with the set off) and see if you find a terminal around half the resistance than that you have the output tube plate connected to. That terminal might be the one to use. If you do change something, test fire up the set and measure the voltages to see if they are closer to correct, and check the cathode current of the output tube (and check that the tube isn't red-plating).