There is not enugh voltage to focus due to decay of the divider elements
in the cascade probably. If you are truly desperate then you can locate
some point in the circuit that gives some 600-1000V AC and build a small
voltage doubler to raise the focus voltage coming out of the cascade or
divider block using two diodes rated 10kV and two caps rated same, plus a
protection resistor that you get to calculate (to protect the ac source).
Note that this is VERY DANGEROUS and it should only be used in cases of
terminal desperation. ;-)

Peter

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