First check whether this problem only occurs when playing a certain game or if it occurs with all games.
If this problem only occurs with a certain game, please determine whether the game supports button control and if the button layout in the game's options menu may possibly have been set incorrectly (for detailed information please consult the respective game manual).
If you use an unlicensed screen protector on your Touch Screen please be aware that this can send incorrect impulses to the device. This may impact on the response of other functions of the Nintendo DS. Please remove the protective film and retest.
If this does not solve the problem, the Nintendo DS will need to be returned to Nintendo Australia for inspection/repair.