I have a 2012 iMac 21.5", which until recently quite happily went to sleep with a brief click of the power button.
Now, it does not.
If I press and hold the power button, it asks me if I want to sleep, log out, power off etc, so the power button is obviously functioning - just not as it once did.
I'm running Mavericks and auto-load any new updates, so am up-to-date on software.
Thoughts?