This might not be your problem, but I've had the same issue for a long time and couldn't find an answer for me, so I'll post what worked for me in case someone comes along later.
My MacBook Pro (and later, my iMac, since I used Migration Assistant) would not sleep when left idle for anything. It would sleep with the lid closed, or if I explicitly told it to, but would not sleep when left alone, as it should have per my preferences in Energy Saver.
I had already checked
pmset -g assertions and turned off sharing and other wake-on-LAN settings, and nothing seemed to work. Monitoring
pmset -g pslog, I found that the computer would go to sleep at the scheduled time, and wake back up a few seconds later, reporting as
wake reason only whatever had been called last—even if it was the power button (that is, it said I had pressed the power button when I had not. That scared me). I had read this and a dozen other answers and forums and couldn't get anywhere.
I found a hint by checking
pmset -g log which was a little more detailed than
pslog. It turned out that every time the computer went to sleep, a process "cancelled" like so:
3/13/13 12:00:12 AM CDT Sleep Software Sleep Sleep: Using AC 6492 secs
3/13/13 12:00:12 AM CDT Cancelled Kernel: Response from WDDMService is to cancel state change
Looking up WDDMService in Activity monitor, it turned out to be a background daemon that was installed by the notorious "SmartWare" that came on a Western Digital External drive. So I killed the process, and the computer went to sleep as it should! Since I don't use the software, I removed it from launchd (which I think it called the .plist? I didn't really know what I was doing there) so it wouldn't appear again. Everything works fine now!