This is always due to a process keeping the system awake, but determining which is the challenge.
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.
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
Good luck!
