Lke a little kid, my Windows 7 machine has started to not want to go to sleep sometimes.
The monitor shuts off at the set time, but lately, more often than not, no sleep.
I tweaked the settings from the default 30 minute to shorter times to see what is happening, and sometimes it works for a few times after that, but then back to its "I'll sleep when I'm dead" attitude.
I actually shut down the machine, and that sometimes seems to help for a while, but, still always, back to usually not going to sleep.
I understand that some usb devices can cause or at least exacerbate the problem, but nothing has changed with my mouse, keyboard, scanner and printer.
I run a solid state C drive, a regular HD for data("E") and and have a removable HD for Ghost backup.
I usually leave my machine on, and so, rely on sleep mode to save a little juice and wear and tear.
edit I gotta admit I'm re-thinking leaving it on with a 33 second boot time
