Although I'm a Power User Hippie working on this issue for a year now,
instead of a Nerd here's my experience...
Although the Powercfg cmd was not the end all answer for me either, It is the first step &
therefore NOT Useless. There are many, many differences between auto & manual sleep.
How are you...
"certain that the solution must be very simple for it already work when manually commanded to sleep."
- That's a bold statement knowing how windows "works."
For me it is/was a compound issue, first the Powercfg cmd showed me my audio driver was the problem,
so I used another one that the Powercfg cmd showed fixed it that time, sleep returned to normal for a
few days, then broke again. Powercfg confirmed it was now another cause.
At the time none of the other things proved fruitful, including the M$ Fix-it program created for this issue.
Second was turning off my router seemed to fix it the next time, but had no effect before the driver change,
so I then left the Homegroup (without un-sharing all the folders as suggested in below noted thread)
& walla it's fixed again - for now.
If you read this near 200 page forum post, you will see lots of people have this problem in some fashion
on many kinds of hardware & after at least 7 fixes worked for some but not all, this tells ME it's not so
simple a problem. Many things prevent auto sleep, the manual activation of it ignores them all, I believe.
Good Luck to all of us "2+ year beta testers" of win8. - I can't afford to eat Apples.
http://www.eightforums.com/general-support/28226-windows-8-1-refuses-sleep-112.html