As a villager approaches master status in a skill, the amount of improvement becomes smaller and smaller with each repetition of the task. It can appear that no improvement is being made, since each increment is much smaller than it was in the untrained/trainee levels. People have reported success in training a master builder using the debris and completing puzzle 12, but I don't think they started with a totally untrained villager.
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Barbara
Unicorn
Last Day of Work