If the 12BH7 didn't glow, but now does, there are a few possibilities.
One is a loose wire, as Grainger suggests, and this would have to be between the 6.3V power supply board pads and pins 4/5 and 9 of the 12BH7 socket. You can check to be sure that these are tight and well soldered by tugging on them a little with your needle nose pliers.
I tend to find that it is more likely with this kit that there are metal bits between pins 4 and 5 on the remaining 3 9-pin tube sockets that are too close together, and potentially touching. This would short out the 6.3V supply and also produce the symptoms you describe.