I know this isn't any help at this point but it is weird you can't get a master.

I do one task at a time, started with 2 builders on hut then up to 3 builders on clearing leaves and 4 once I got to clearing the rubble. I have been spreading out the work with my other villagers as well so they can earn skill in all levels working towards Elders.

I have 4 masters, 3 under the ages of 30 and 3 adept under the age of 25. They were all adept pretty quick into whichever building task I started them in first. Only one of my master builders has done nothing but build, He has been a master for a long time and he just turned 50.