My Mac Mini won't go to sleep
To debug this, you'll need to open terminal.app and look at the power assertions log. It's pretty verbose, but it really shows what's going on to prevent a sleep from happening.

pmset -g assertionslog


If you need a smaller set of data to look at, just ask for the current status and not an ongoing log of what's set and what's changing.

pmset -g assertions


You can close the window when you're done with the tools.