It is possible that gas has formed inside the tube, what is recommended in these cases is a procedure called "seasoning", in the most modern equipment, they can already do it with a simple eating, in the case of your equipment it has to be done manually: you have to start making short shots, with low techniques with both filaments, 4 or 5 shots with each technique. Gradually see the technique going up, first the 40, 60, 80, 100, 150, 200 with each filament then raise the KV 100 and start again with low mA.