I am going to guess that the problem with your ticket was that NW still had control of it. All that means is that CO needs to pull the ticket over to CO's side, to show that you are holding an e-tkt.

There is no reason to make you wait because you are holding a reservation and it's just an internal ticketing function that can be done later. But if the agent is not use to seeing this or has never seen this than I can see where they might question you about it.

If this ever happens to you again, instruct the agent to call CO Res and they can do the research while you wait and fix the problem for the check in agent.

If you do this again I strongly suggest you contact CO Res a few days before you leave for your trip, to make sure that everything is ok on the CO side. We have a very good Help Desk in Res.