Courtesy RCG user MalcolmHolt
The default throttle trim is set to ATL mode in which the trim lever only affects one half of the servo’s travel. This allows you to fine tune the tick-over speed without affecting the top end speed. It sounds as if your ATL is operating at the wrong end.
To fix this go to the Function Menu and scroll to the throttle trim (T3 on Mode 2) and tap RTN. In the bottom right-hand corner of the screen it will say either Normal or Reverse. Whatever it says change it to the other one.
Alternatively, if you do not want to use ATL, change Mode from ATL to Normal.