unplug the network connection and see if it will sleep.
often the pc will act as a media extender and will talk to other devices on the network (other computers or a xbox)
you can also see what is blocking sleep functions by running cmd.exe or powershell as an admin then run
powercfg.exe /energy
and look at the report.
could also be your Ethernet setting for power management.
use control panel, find the device and right mouse click on it. make sure it is set to allow the device to sleep and wake on LAN is not set.
(disconnect from the network to test to see if the machine will sleep)
media servers on your machine can prevent your machine from sleeping.
(should show up on the powercfg.exe report)