[iBook] iBook not sleeping?

[Concetta Zito]

1.2MHz G4 iBook running 10.4.5
I don't think I am able to get my iBook to sleep on its own (after a  
specified time of inactivity).
If I choose "Sleep" from the Apple menu, or if I close the iBook, the  
familiar white light indicator turns on and starts pulsating.
BUT
I have my screen saver set to come on after 3 minutes of inactivity.  
It does.
I have my Energy Saver system pref set to put display and computer to  
sleep after 8 minutes.
After 8 minutes, the display is dark, but the computer does not go to  
sleep (no pulsating light, and just touching my trackpad awakens it).
What am I doing wrong?
Thank you.
Concetta