Yaw stabilized performance is bad when in transition mode with tilt rotor vtol plane

I have created a new airframe with four rotor and the two front rotor can be tilted.
When I tested it, I found the yaw angle can’t be stabilized when the two front rotor tilt angle is bigger than to 9 degree.
I have no idea to this.
Whether I can solve this by tuning the pid parameters?
any one has some experience on this?