Mac doesn't go into sleep mode Sometimes my mac mini doesn't go into standby after the inactivity time specified in system settings. This happened with Yosemite, and continues to happen with El Capitan.

Quite sure, there is some application that prevents sleep, but I'm still unable to identify it. Is there a systematic way to identify the guilty application?
After trying everything i could find on the web, i figured out that it was a page in the (HP) printer queue that was keeping my Mac from sleeping for days. Turned on the printer, forced the page to print and my Mac is now FINALLY going back to sleep.  

There was no indication of this in the Activity Monitor nor the "pmset -g assertions" command that pointed to anything printer related. Hope this helps someone !