I have had far better luck in getting Delta rewards than NW awards. Try routing through CVG, if possible, because there always seems to be more award availablility out of CVG than ATL.

Also, if you don't mind an extra stop, try a combination of two carriers. Last year I booked a CVG-DTW-SLC-ELK-RNO trip, part on Delta, part on NW, part on Skywest.