Pode estar faltando o driver do chipset, tente instalar os drivers de chipset mais recentes da sua placa-mãe e verifique-os.
Além disso, se sua placa mãe é da Biostar, você pode verificar a seguinte questão
How to use keyboard or mouse to power on or resume the motherboard?
If use USB keyboard or mouse, press Delete key in POST screen to go CMOS> Advanced> Power Configuration> USB wakeup From S3/S4 set this function Enabled. Also need to set JSUBV1 and JUSBV2 to pin 2-3 for motherboard to provide 5V stand-by power to USB devices for resume.
If need to power on motherboard through PS2 keyboard or mouse, press Delete key in POST screen to go CMOS> Advanced> SuperIO Configuration> Keyboard PowerOn / Mouse PowerOn. Set these two functions to Enabled. Save the CMOS setting and make sure system reboot once to POST screen then use power switch to turn off system power. It will be able to use keyboard or mouse for power on at S4 or S5 condition.
Note: Turn off PSU power source or unplug the power cord will cause PS2 keyboard or mouse power on function fail. This function will be able to use again after system reboot once. src= http://www.biostar.com.tw/app/en/support/faq_content.php?S_ID=459