missions question

been playing wc3, am level 72, have beaten most of the campaign up to aliens, have some good generals, etc, doing well in this regard. lots of wonders opened, including the terracotta army. have beat all of 1939 and 1943, working on 1950.

my question is that for the past week or so, i no longer get red missions, only yellow ones.  is this because i need to open more of the game or did i finish all the red missions?

thanks.