Looking at the model's mesh where the tail joins the body, is that a homogeneous area or has the tail been "stuck into" the body?

Try moving the twist start point away from the body.

For the end of the tail, use the Joint Editor to set the orientation of the center such that the axial rotation aligns to the tail. You may be able to do this by simply correctly positioning the center and end points and clicking the Align button. If not then use the orientation dials.