Sounds like just an unlikely game -- villagers don't gain skill every time they are successful at a task, they gain it only sometimes...and the better they get, the less likely that they'll gain more skill, I believe.

You just need to keep trying, and keep the skill check at builder -- call it bad luck like some of the other threads where people have villages with lopsided numbers of boys vs. girls because of random luck with breeding.

There are also a couple of other buildings she can build to try and get that master rating.

-- jen