Guns.com has Top 12 reasons for NICS denial in last 15 years. Out of 183 million checks so far, about 0.6%(about 1 million) resulted in denials. Half of denials were due to criminal record, and some were denied for renouncing citizenship.
You can read more from:
http://www.guns.com/2014/02/15/top-12-reasons-fbi-nics-background-check-denials-past-15-years/
From: Guns.com
