Hmmm, this is odd for sure. You said you performed a "clean start". Did you mean "
Clean Boot"?
Do you keep any attached devices connected to this notebook?
I ask because I had a "similar" problem on one computer here where the computer never went to sleep. It tried like it was supposed to but immediately rebooted. But I could tell it to shutdown, and it would with no problems. And the problem started only after updating to Windows 10 from W7. It took awhile for me to finally figure out what was happening - though I still don't know why it was happening.
To more or less fix (it is more of a work-around) the problem I had to go into device manager and for my mouse and keyboard under Properties > Power Management, I had to uncheck "Allow this device to wake the computer". As soon as I did that, 15 minutes later, the computer went to sleep as it is supposed to.
Now again, I said "similar" problem and not the same because I was able to shutdown the computer. But you still might look at some of the devices in Device Manager and see how they are set. Check your network devices too.
Do you open and close the lid and move the notebook from place to place regularly? I am trying to figure out why it worked fine for several days then started giving problems again. Working/not working as it is moved about could support the loose connection/pinched wire theory.