Could hibernate only on battery


I am using openSUSE 11.1 with all official updates on a notebook ASUS F5GL. My kernel is: Generally power management works fine - I see battery status in kpowermanager, I can suspend ( to RAM ) and restore.
I can also hibernate when my laptop is on battery but strangely, if I have the power plugged in, the hibernate only restarts the machine.
My machine is in the s3ram whitelist, I checked /var/log/pm-suspend.log but everything looks normal:

2009-09-29 22:18:33.515104822: done running hibernate/hibernate hooks.

  • /usr/sbin/s2disk --config /var/lib/s2disk.conf
  • RET=0
  • set +x
    2009-09-29 22:19:22.698872462 +0300: running ‘hibernate’/‘thaw’/‘reverse’ hooks.

I checked thoroughly all suse forums and related wiki but I could not find any similar issue or idea what could be wrong.
Does anyone has a clue what could be wrong and how to fix it ?