[Dekatron]

Have you rotated the Electrode Selector switch, the Circuit Selector switch and pushed the two buttons mA g2 and mA/V g2 to see if that "frees" something up?

Can you do an ohms check across pin 1 and 2 on the amplifier board to see that they aren't shorted?

I have seen broken wafers and broken center pieces on the rotary switches making a mess of the measurements as they sometimes become open circuit and in some cases permanently short some positions.