[brazzledazzle]

You've probably already looked but console might actually tell you exactly what the culprit is. This has happened to me in the past a couple of times. Once with an app running in the background that I can't remember the name of that wouldn't even let the screensaver start, much less sleep. The other time was because of a kernel extension shitting the bed on a regular basis. If you're curious what non-Apple kexts you have installed you can see them like so:

kextstat | grep -v com.apple

They're at /Library/Extensions/ and you can disable them like this:

kextunload /Library/Extensions/Some3rdPartyModule.kext

Even if it's not a kext it's good to keep tabs on kernel extensions you have installed. I hope that helps and apologies if you're already aware of all that.