I had the same problem with two of my flights.

My ticket was purchased on NWA.com BUT some of the flights are "Delta" metal.

I called NWA.COM tech support and the woman explained what I had to do.

You have to get the DELTA Confirmation number from the the reservationist or elite line and THEN go to Delta.com to change seats there. Your elite status will be recognized there for premium seating.

So you have to go back and forth between NWA.COM and DELTA.COM to make changes to your entire itinerary.

I asked the tech support woman how long this will go on and she said she had no idea. She was about to make a guess - thought better of doing that - and said - "We really don't know how long this will be."

I hope this helps in your situation.