Second RFE

My hu*****and is the main applicant. We filed his I-140 and our I-485 in November 2004. He received his green card in September 2006 while mine is still pending now. In October 2006, NSC sent me a RFE for marriage certoficate and I submitted it soon. I received an email last week from NSC asking the second time from RFE.

Anyone knows what is wrong with NSC? The letter is not here yet. I could not think of what the NSC would ask for this time.

Thanks for any guess.