Screen saver stops activating on Karmic

[skx skxinski]

I have screen saver set (System -> Preferences -> Screensaver) to blank
and lock screen after the computer is idle for 2 minutes on Ubuntu 9.10
Karmic Koala. 

After some time since the last reboot or hibernation the screen saver
just doesn't activate anymore, no matter for how long the computer is
left unused. 

It seems to happen after I "catch" the screen saver in the darkening
phase a few times and press a button or move the cursor to stop it from
coming on but I'm not entirely sure. 

What can cause this weird behaviour and how do I pin it down? 


-- 
skx.