There currently is another thread addressing this issue: http://www.flyertalk.com/forum/showthread.php?t=697489

This is a common problem with co.com and has been going on for quite some time. For that reason, you should always call to verify availability and book reward tickets rather than rely on what is listed on the website. In addition, the first person with whom you spoke should have told you to have the elite line book the reservation rather than redeposit and try again on co.com. That would have made more sense, IMO.

It is unlikely that CO will open up reward space for you as a goodwill gesture, especially given that the flight is operated by NW. Good luck getting your issue resolved.