Why is the screen saver not kicking in? I have  a Mac mini running 10.6.7. I have set the screen saver to be automatically activated after 15 minutes, and the session to be locked a while after that.

But sometimes, the screen saver stops working. The screen remains on and unlocked indefinitely.

I sometimes suspend the machine manually (for example by pressing ⌘-⎇-⏏). This will suspend it properly, but when I come back to it sometime later I will often see that it is on again and no screen saver.

This problem appears after I have been using the computer for a while, and then doesn't go away until I turn it off (or perhaps just log out). It could perhaps mean that some screensaver process has died and not restarted automatically.

Can anyone help me figure out what is going wrong, and what I can to to fix it?

Update: In the end, I found the application causing it to not suspend. It was SqueezePlayer. I didn't any really good suggestions how to find the culprit, though.
I cannot tell about screensaver for sure, but you can use pmset -g to show which process is disturbing sleep (think this first appeared in 10.6.7)

# pmset -g
[cut]
sleep       0 (imposed by 1517)
[cut]


this means that pid 1517 is preventing the system from sleeping.

# ps ax |grep 1517
1517   ??  Rs     1:59.76 /System/Library/CoreServices/backupd 


This shows that this is the backup daemon running.