Not all NM windows open after spawn.
Take Shikigami Weapon. Spawns 20 hours after maint.
Argus is probably similar.
Btw. I'd suggest doing Nyzul Isle floors for Argus really. We had him a total of 3-4 times today in Nyzul Isle, and in the 1/2 times we killed him, we got a PCA. =x