I had unlocked all the others by noon yesterday, but the last one never opened up. I still kept track of my damage, in case it was a Secret Mission, but even after 300 damage, nothing happened. I've contacted customer support, I guess that's all I can do.
I think it may have to do with the timing.... you may have unlocked it in time if you go by your local time, but the mission might have been closed by UR office time (in France i imagine)
UR needs to set an option where we can all see what the current server time is..... if this option doesnt exist yet it should become an option