I created a vassal merchant republic in my kingdom. It is now several decades later and the merchant republic seems to own zero trade zones. The republic is in the Baltic and there are no competing republics in the area. Also, AI republics in the Mediterranean (Venice, Pisa and in my current game Lukania) have generated wide spread trade zones.
I have looked at two of the heads of the patrician families of my vassal republic and they have plenty of cash, certainly enough to build trade zones. Since the AI has plenty of money, this should mean that the reason for not building trade zones is not that it's prioritizing other construction. Would the AI save up money for something before it builds trade zones? Are there some conditions the AI must fulfill in order to start building trade zones?