let SetMonitorBrightness return FALSE

Hi,

I am developing a application of monitor brightness control on computer with windows 8. The monitor has support DDC/CI, So when I used SetMonitorBrightness(), it always return TRUE.  What I need to know is, in what conditions, the SetMonitorBrightness() will return FALSE. MSDN didn't describe this very clearly. it just said "If the function succeeds, the return value is TRUE. If the function fails, the return value is FALSE ".

Does somebody know detailed information about this problem?

Thanks in advance for helpfull answer.