Assuming that you’ve set Windows 8.1 to sleep in the power options, Windows 8.1 might not be sleeping when idle because of either a BIOS setting disabling the computer from sleeping or because you don’t have the right drivers for your motherboard.
You might also try the following command prompt:
Checking the energy report might give a hint about the reason that your PC won’t go to sleep.
Also, try updating the BIOS firmware. You may have trouble with Wake On LAN, wake on USB or some other setting that’s stopping Windows 8.1 from going to sleep when idle. Try removing it from the network or removing USB devices to confirm this. The following command shows a list of devices that are capable of waking up a PC (and therefore, potentially able to stop it from sleeping):
Powercfg - devicequery wake_armed
Also, having the system set up to defrag when idle could prevent Windows 8.1 from sleeping.
You could also go through the Device Manager and check the properties on all your devices to see if any are set to be able to wake Windows 8.1.