Troubleshooting:
(Assuming the computer is configured [rather quickly for testing; or have a sleep corner active] through the Energy Saver preference pane...)
Unplug all USB devices including HDs and hubs to try and discover the culprit preventing sleepification.**
Drag your 'Energy Saver' preference pane [mainHD/Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist] to the desktop, and try to sleep the machine.
(reconfigure the Energy Saver preference pane (or place back into its folder, if this didn't work....)
(** ®©℗℠™fakegeorgeW.bush)
