Ok, I've been following this for a couple of days now and I have a couple of questions...
Do you have the turnbuckle links connected to the steering knuckle? I ask because the ball ends are a bit tight and should be polished with some Scotchbright prior to assembly.
Was there any binding in the c-hub without the BH screw and/or ball screwed in? Like the c-hub is clamping the knuckle.
The screws usually need to tap into the plastic which will provide enough tension to keep the screws from backing out of not fully seated. (dont fully tighten them until the parts break in)
This may seem like an obvious no brainer but did you insert the bushings from the inside of the c-hubs? (no disrespect intended)
I've built a lot of cars over the last 20 years or so, myself and these are just some of my observations while building my TC6 last year.

The bushings are for locating the screws but may not extend beyond the c-hub opening otherwise there will be extra play in the steering knuckle once the plastic has worn in requiring shims.
"Better tight from the start than sloppy for life" momma always said.