iBook G4 won't sleep a la Energy Saver!

I recently noticed that it won't sleep automatically even though I have the Energy Saver settings set to have it sleep after 10 minutes.
Lot-o-fun wrote on :

I recently upgraded my iBook G4 to Tiger and then to 10.4.1. I recently noticed that it won't sleep automatically even though I have the Energy Saver settings set to have it sleep after 10 minutes. I tried deleting some preference files (/Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plis t and /Users/me/Library/Preferences/com.apple.systemuiserver.plist) and restarting, but that didn't help.

Any ideas where I might look to figure out why this is happening?

TIA, -Lotofun