Many IN-9 tubes needs to be "formatted" when first powering up. I will add a paragraph to the manual. As you have just assembled the thermometer do the following peocedure:
Put the NixieTherm into a warmer ambient (around 25°C). Now turn the potentiometer on botton for max. display. If the glow will not reach the 30°C marking than keep the NixieTherm turned on in this ambient until this marking is reached. Than the formatting is finished and you can re-adjust the potentiometer for correct display.