I've occasionally run into this problem with DL tickets, especially if I've tried to find the flight using the DL PNR that NW assigns to it -- somehow my DL FF number, which is automatic when I log on to the DL www site. And, when I cal DL, I usually talk with someone out of the country who doesn't quite understand what I want to do.
The only airline that I've ever encountered that can seem to manage two FF numbers, one for logging in to their www site and one for crediting FF miles is CO (maybe NW handles it as well, but since I always credit my NW filghts to NW, I've not encountered a problem).
tom