I had to do this once. Sometimes it can just be an erroneous denial.
You can file an appeal with the FBI (not the ATF) , and request the reason for denial.
Go here--------> Federal Bureau of Investigation - NICS E-Check
Check out the site, you may request the reason for your denial by writing. Instructions on webpage. After you get the reason for denial, (wrong information, right information/wrong person, etc) you can then plan out your next course of action.
I think it took me around a week to get my reason back the one time I had to file an appeal.