Septimius
Warlord
Been having a bit of internet troubleand today is all appointments.
edit My quick look showed that the mission was missing off the unit in the Slavery mod but those units have been moved out of the Slavery mod and into core since you can and should get captives even if you aren't playing with Slavery on. The mission is correctly defined on the core unit, so I am not sure what is going on. I'll need to test in game to see if I can replicate the problem.
Ok, that's another guy's problem. I can take prisoners no problem, but the captive-civilians don't get the option to increase the city size by one, when below size 7.