I have a CW voltage multiplier that uses 30kV, 1nF capacitors and doubled 20kV, 5mA, 100ns diodes (for a total of 40kV). My ac source is a sine wave (viewed on the primary side of the transformer because the secondary is beyond the capacity of my oscilloscope) around 20-24kHz and produces 15kV DC when half-wave rectified (measured after the 150Mohm resistor.)
The output voltage of the CW multiplier is only 1.5kV. I have tested the diodes and capacitors. 100ns is compatible with most frequencies below 1MHz, I believe.
I am thinking that it might be perhaps that the 12 \$\Omega\$ resistor is not allowing enough current to charge the capacitors, but maybe it is some thing else such as some reactance I am not taking into account?