I have been having that same problem trying to create master builders when there are no more buildings to build and the other building projects (I don't want to creat a spoiler) are completed as well. In my very first village, I had plenty of master builders fairly quickly, but I don't know what I did. In my second and third villages, I have 2 (in the second) and none so far in the third. Wish I could get master builders are in my other villages as I did in my first. And wouldn't you know, it was before I even discovered these boards and had no clue as to whether having master builders was important or not! LOL!