I presume that you have done this already but I thought I would suggest it anyhow. All these villagers who are not becoming masters, do you have checkmarks in the DETAILS for them to be builders. I sometimes forget and wonder why, after some considerable time, they are not at least trainees yet. When I go into the DETAILS, I see the builers skill bar is way up there but that the checkmark was for something else that had no bar at all.