Unlocking specific city missions during invasions

Hey, I'm loving this game but even through trial and error can't figure out how to always unlock the Prison Break option when it is available. I can get the Archives to open every time, but no amount of city exploring or warlord espionage is changing what I assume is the Free Prisoners mission from locked to unlocked in most situations where it appears. I've even had 2 warlords double team a city repeatedly completing military/domestic missions stated to unlock additional options to no avail. I even maxed out city intel level to 100 while doing all of the above and am out of ideas. I'm sure I'm missing something but really want more warlords from whereever I can get them. Any help would be greatly appreciated!