Hi Steve,
Is yours booking class L? And are your flights towards the end of July?
My wife is a GE and she also has the same problem for her trip in July on NCL > AMS.
I also have an S class ticket between NCL and AMS at the end of July (this is for a business trip, hence the full fare), and had no problem getting 2C.
They are changing flight numbers on this route. I wonder this may have something to do with the change.