First try an SMC reset:
- Shut down the computer.
- Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
- On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
- Release all the keys and the power button at the same time.
- Press the power button to turn on the computer.
Chronic insomnia is always due to a process that is keeping the system awake. Determining which is the challenge. Troubleshooting can be very time-consuming as you make changes then wait while you observe their effects.
- A first simple step is to create a new, temporary user account. Log out of yours and log into the temporary one. Determine if it sleeps then. If it does then you have a login item that is preventing sleep. System Preferences > Users & Groups > Login Items.
- Whenever you suspect a problem related to power, including sleep, an SMC reset is also recommended.
- Active Spotlight indexing will prevent sleep. Wait for it to complete.
- Energy Saver "Wake for network access" can prevent sleep, but unchecking it will prevent the ability to remotely access a sleeping Mac over a network. A normally functioning Mac will sleep according to Energy Saver, but will wake periodically for network access, after which it will return to sleep according to the settings in Energy Saver.
- Unfinished print jobs will prevent sleep. If a print job is queued, but if the printer is off or the connection to it is lost, the Mac will stay awake forever waiting for the printer to return.
- Check your Sharing preferences for anything that could permit an active network connection.
- Safari pages that periodically refresh themselves are very common. This will prevent sleep.
- Frequently checking for new mail may prevent sleep. Change Mail's preferences to check for new mail less frequently.
- iTunes and iPhoto sharing will prevent sleep.
- Active Bluetooth devices will prevent sleep.
- Active USB devices will prevent sleep.
- Using Time Machine over a network can prevent sleep.
- There are plenty of third party utilities designed to prevent sleep. One may not have been completely uninstalled.
- Quit the process with the name powerd in Activity Monitor. It will re-launch on its own.
- Any number of "anti-virus" utilities can prevent sleep, along with other miseries. Get rid of them.
Mac OS X: Why your Mac might not sleep or stay in sleep mode: http://support.apple.com/kb/HT1776