Hibernation doesn't work

When I select Hibernate from the start menu the screen just fades away then after a second it comes back.
I have tried disabling hibernation (powercfg /hibernate off) and rebooting then re-enabling hibernation, but hibernate still doesn't work.

Welcome
Go to search, type device manager. In device manager look for any yellow warining markers that some drivers may not be working properly, update drivers.
This tutorial may also help
http://www.vistax64.com/tutorials/63567-power-options-sleep-mode-problems.html
http://www.howtogeek.com/howto/windows-vista/re-enable-hibernate-option-in-windows-vista/

There are no device driver errors shown in the device manager. I went through the steps in your link but didn't find any solution.
S3 sleep works like it should, btw

You might want to look in this tutorial for additional help!

http://www.vistax64.com/tutorials/74022-hibernate-shortcut.html