I haved solved all but puzzle 12, I have 'Master's' at everything but construction. All there is to do is to clean the beach and that doesn't seem to 'develop' the building skill beyond 'trainee' Any suggestions??

The only way to train builders after the first 9 puzzles are complete is the beach debris. I know of several people who have managed to use several "batches" of debris to train another master builder and complete puzzle 12. That situation will be corrected in the v1.1 update, but the release date has not been announced for it yet.

Actually all my master builders got swallowed up by the earth when I tried to get them to work on puzzle 12; later (after the birth of the Golden Brat), I noticed that one of my adept builders had become a master builder... all he had ever done was wander around fixing huts periodically, but apparently it was enough to upgrade his skills over time....