A few ideas :

Looking from the top down above the hub, make sure that the hub is true when turning at slow speed, the small phillips screw should not wobble around the main shaft axis,
Make sure that the blade grips are not too tight on the blades, the blades should be easy to move in the grips,
Make sure that the spindle is not moving too freely in the hub : the o-rings and the shims should allow some movement but not much,
Make sure that the bell mixer arm screws are tightened just enough so that the flybar can flap freely,
Try balancing the rotor assembly without the blades, then try balancing the flybar, then the whole thing with blades.

Good luck,
Georges