I ran into this problem before as well. I was told (by OnePass) that if seats are available for reward travel on NW using WorldPerks miles, it does not necessarily mean that you can get a reward seat on the same NW flight using OnePass miles. Apparently, WorldPerks and OnePass have their own, separate allocation of reward seats on each NW or CO flight. I don't know if this is in fact the case, but it was what I was told.