Probably not counterfiet.

So far there are no known cases of counterfiet ngc slabs. Now, as with all things it takes a little while before a coin that has been slabbed recently to be added to the database. I would contact NGC if you haven't already seen something saying they're checking on it. I'm sure they'll sort it out for you and get your coin added to your set.

I ran into the same problem with one of the first few coins I wanted to add on the day I joined the registry. Within 24 hours NGC admins had worked it out and added the coin to my set for me.