System does not hibernate; the screen flashes off for a few seconds and nothing else happens

I just upgraded my Ubuntu 11.10 to 12.04. I have a problem with hibernation in my new 12.04.

I run the following command

sudo pm-hibernate

but my system does not hibernate, i.e. my system does not turn off; rather the screen just flashes out for few seconds and resumes previous state.

How do I fix this problem and enable hibernation on my system?