Most likely the caps aren't getting fully charged due to the impedance mismatch. That's way far out from optimal. Probably a lot of reflected power there, I don't feel like doing the SWR calculations though.
Can you modify your tank to as close to 11.4nF as you can get? At 9kV input you only need a voltage rating of; 9 x 1.414 x 1.5 = 19kV. If you trust those caps' ratings that'd be about 4caps in parallel iirc. If you can do this then give a try.
JavaTC confirms my hunch that a pancake primary of 5turns of ~10ga with a 6" ID and 10" OD will resonate near 550kHz. Yes I actually took an educated guess on params and it worked out, lol. Does that mean I'm doing this too much >.> ?