Hi,
In a nutshell: try checking if there are any background processes running which stops your computer from going into idle mode.
I had the same problem on my laptop and thought it was a bug in Windows 8.1. After having a good think about the problem I finally sorted it out myself. On my computer it turned out that there was a background process running all the time, the process should
have stopped when not in use but it didn't. I had bought a new OKI printer and installed the OKI PrintSuperVision (PSVEngine.exe) printer software that came with it. Unfortunately for me PSVEngine.exe was buggy and was not compatible with Windows 8.1, but
I didn't know that at the time. I uninstalled the OKI PrintSuperVision software and rebooted the computer, et voilà, problem fixed! The Windows screen saver and all the power options are now running perfectly. The OKI PrintSuperVision software has since been
updated and is supposed to be compatible with Windows 8 (x64), but I haven't tested it yet.
I found the running process by going to: Control Panel > All Control Panel Items > Administrative Tools > Resource Monitor > CPU Tab. It showed that the OKI PrintSuperVision software was still running and consuming about 3% CPU usage, even
though it had been terminated? It was stopping my computer from going into idle mode, therefore the screen saver and the power options just wouldn't kick in.
I've tested it and my computer is now working perfectly even after a number of Windows updates.
Hope it helps.
UPDATE:
Based on my experience, I personally think this issue is related to a hardware/driver/software problem rather than a Windows 8.1 OS bug, although some people disagree.
SYSINTERNALS
Microsoft provides a bunch of troubleshooting utilities called Sysinternals which may help trace the problem, the information is here:
Windows Sysinternals Suite