Did you put in your OP gold elite # for the reservation? If not, they may not recognize you as an elite passenger. Also, seats are only assigned 90 days in advance of the flight. You don't mention the dates of travel, perhaps they are too far out to get a seat yet.
Good luck
Yes, put in OP# - no problem selecting premium seats on flights on NW metal. Flight dates are over the Thanksgiving period, so coming up quite soon.
I wonder if this issue exists on KLM flights that are NW codeshares when the KLM portion is directly out of a US airport? Or maybe it's just specific to certain routings out of AMS (on which KLM may frequently substitute different plane types as demand dictates)?