After updating to 8.1 there appears to be some issue with the function Home Group/file sharing stopping my machine from auto sleeping.
This worked for me.
Disable IPV6 on the network adapter.
Restart the machine.
Re-enable IVP6 on the network adapter.
My PC then auto sleeps and wakes OK until it is shutdown and restarted, which seems to reset it back to the"non sleep mode".
IPV6 is a Home Group requirement, some people have found disconnecting their network card or leaving the Home Group has the same effect. This problem is all over the forums.
Chalk up another one to boys from Redmond.