from http://support.apple.com/kb/HT1776 (Why your Mac might not sleep or stay in sleep mode from apple support)
Open applications can affect sleep
Software that accesses the hard drive will delay sleep mode until it is either quit or done accessing the drive. For example:
- Songs playing in iTunes.
- A movie disc playing in DVD Player.
How can I determine what is causing a sleep issue and how can I resolve it?
These are the most likely causes and solutions for sleep issues:
- Sharing preferences: Enabling sharing for any connections can potentially disable sleep. Turn off file sharing if it's not being used.
- Bluetooth settings: Bluetooth can wake your computer if you set the "Allow Bluetooth devices to wake this computer" checkbox in Bluetooth System Preferences.
- Are a keyboard and mouse connected and working correctly?
- Do you have any other external USB or FireWire devices connected? Try removing or disconnecting them one by one, when not in use, taking care to dismount any that are storage devices before disconnecting them.
- Do you have a print job in the Printer Queue? Remove any print jobs that are in the queue.
- Is Spotlight indexing? This can take anywhere from several minutes to several hours after a new installation, or after connecting a storage device which contains unindexed data. Indexing prevents idle sleep.
- Additional Software: Try a Safe Boot to help isolate a software issue.
- Put your ear near your computer. Do you hear the "clicking" sounds of hard disk access?
- Energy Saver settings: If the computer and display sleep timers are set to "Never", the computer will not sleep.