Have you had the head tached?

Are you sure your flybar/paddles are in balance?

What I do when setting it up: Insert the flybar rod and measure to the exact center, and THEN I'll see if it balances by watching it teeter on the mast. (NOTE, I do this with the cage links dangling and NOT connected to the mixing arms). Tighten the rod to the cage.

I then apply the weights if desired and temporarily slide them in towards the cage. From there I'll screw on the paddles at the same distance out from the cage, Then I'll check to see if it balances (teeters evenly on the mast).

Slide your weights out to the desired distance then check the balance ONE MORE TIME. Finally you can connect the cage links to the mixing arms and you're good to go.
BTW, I balance my tail blades too. EVERY Rex heli I own has virtually NO shake/vibes. FWIW, if it spins... balance it.

Good luck