This happened to me a few months ago. I though it was related to some sort of update to the OS that I had run but wasn't sure. Anyway I just got used to waiting a bit longer before I walked away.
Here is an Apple article on the subject which may give you some ideas:
http://support.apple.com/kb/HT1776QUOTE
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.
However, this can also occur in subtle ways you might not expect:
If an application uses a custom font to display text but the font resource is marked purgeable, then at some point the memory manager will purge the font from memory. When the application tries to draw text using that font again, it will be loaded from the hard drive, resetting the sleep timer.
Applications can be designed to keep the system awake and prevent idle sleep indefinitely. In developer lingo, an application may explicitly prevent system sleep by calling IORegisterForSystemPower(), and calling IOCancelPowerChange() when it receives a power management kIOMessageCanSystemSleep notification.
Also this thread has a Terminal input that can give you more detail:
http://forums.macrumors.com/showthread.php?t=776356Of note, several people in that thread found old print jobs in the queue which immediately solved their problem once they deleted said job.
