Availability for award tickets comes out of special fare buckets for awards: on NW they W for coach & A for first-class. Availability of paid fares has no bearing on the availability (or not) of award fares. When standard awards are unavailable, you can usually (but not always) obtain a "rulebuster" award for 2X miles.

NW has additional availability for its elites. You will only see this if you sign in while you are searching for award itineraries; this is a separate login from the one for your account summary. Some partner availability (esp. CO, DL, KL) will be shown, but for best results you might need to have a phone rep do the searching & booking for you.

CO's main hub & HQ are in Houston.