I built one of these for a UK magazine review, some 3 years back. I know the problem you have well. There is a manufacturing problem, and to my knowledge it's never been sorted.

First off, you have to accept that you are never going to get much rudder movement. It will be controllable on the ground, but never as much as you would really like. Next you are going to need to remove a small part of the fuz, at the back, under the tail to help gain so extra movement.

I'll see if I can dig out an old construction shot and post, to show you what I did.

One other small point, she can be a bit-of-a-pig! on the ground [&o] but in the air..a dream leave it with me for a couple of days and I'll get back to you.