I am running into a building problem. I have several large towns with a wooden wall. When hovering over the icon I get the message "Wooden Wall (upgrade)". When entering the building window there is no Stone Wall building option (net even greyed out). There is no indication why it is not possible to build a Stone Wall while the city is a large town.
I tried to find the requirements or bugs for the Stone Wall and only found that is requires a large town.
What am I missing and are there hidden requirements for building upgrades?