Please calibrate the Touch Screen. Choose the option "Touch Screen" in the Nintendo DSi or Nintendo DSi XL settings menu. Please make sure you exactly tap the displayed calibration points (upper left corner, lower right corner, middle). If you have done this correctly a message "Calibration complete" will appear.
If you use a protective film on your Touch Screen please be aware that this can reduce the sensitivity of the Touch Screen. In addition, if the protective film is not attached properly, the Touch Screen can receive incorrect control impulses, which can influence your ability to control.
In this case please remove the protective film and re-calibrate the Nintendo DSi or Nintendo DSi XL as described above.
If this is not successful the Nintendo DSi or Nintendo DSi XL will need to be returned to Nintendo Australia for inspection/repair.