Make sure you have enough Latifundae.
That seemed to be the problem in my campaign. I think I had to built more than the number the FAQ said (though that may have something to do with the City Mod and some changes I made to the City Mod to get the March of Time working...).
use "add_population 4000, add_money 40000 and process_cq City_Name" to build them quickly.
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