Mac doesn't go into sleep mode
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 !