Saidas,
have you verified what devices you have set to wake the machine up? most often it is a Wake On LAN setting the keeps computers up, but these days, with Multimedia PCs and wireless devices being all the rage, it could be a multitude of things.
For example, on my computer, I have 2 wireless mice (an old Logitech Trackball Optical Wireless and a new MX Revolution wireless) both of which are set to wake the computer on activation - I have found that turning off the MX and removing the battery from the trackball keep my computer asleep when it is supposed to be asleep. I also have a eHome Media Transceiver for use with my Remote Control, but that is passive, so it does not interfere, and in fact, the remote has a sleep button on it so I can enable sleep without having to be at the computer.
I believe the command for checking sleep devices is in the tutorial that Brink mentioned a few posts back.
One other thing - depending upon the number of processes you have running and the amount of RAM you have, it may take up to 30 seconds for hibernation to kick in as it is saving all the stuff from RAM to the hibernation file.