There are three security checks for the N-400
1) FBI Name Check
2) IBIS Check
3) FBI Fingerprinting
Are you positive your name check has cleared? I know someone who has been waiting for 2 years for the oath letter but their name check is still pending and they are preparing to file their lawsuit.
Just to be on the safe side, I would take an infopass and find out what the problem is. The IBIS check is an intra-border security check possibly cross-listing your name on other foreign security agencies to make sure that you are not wanted or have committed a crime elsewhere.
Have you renewed your GC recently? Both IBIS and FBI Fingerprinting need to be complete before issuing a new GC, however the name check is only for the N-400.
Hope this helps, there is an article somewhere which discusses the various security checks for all immigration benefits, if I come across it, I will let you know.
Good luck!