I don't have experience of running neons in series - I'm not sure if that works fine. I have always run them in parallel in the past.
The reason that I think it might not work, is that neons are high volume, low price things with wide tolerances. I can imagine that if they are not well matched, one neon starves the other. I'm not sure, I'm guessing.
Also, in the clocks, we are running with PWM, which is also a bit of a challenge for neons. They don't always ignite predictably.
Are these neons all the same, or are they mixed types?
Ian wrote: Try them in parallel? With a 180k resistor on each?
I didn't notice that he said he's running them in series. I agree they need to be in parallel. I'm surprised they light up at all in series. When I suggested 68K I was thinking of 1 resistor feeding 2 parallel neons.
Look into it later when the dust is clearing off the crater.