Sometimes it just takes a little longer when your ETKT gets sent to a queue that is processed manually by the ticketing dept. with priority given to those tickets with departure dates sooner than later. I've had to wait several weeks on a Revenue ticket purchased with a CC to get an ETKT number.

So long as your flight is ticketed by the ticketing department prior to departure, you're fine. So a close eye on your reservations is always good.

OTOH, if you're truly nervous about it, it never hurts to call NW and have the agent call the ticketing dept. to force the ticket through. Most agents aren't willing to do this, but one may.