This is driving me nuts. I've gotten the crate on the beach open with my team of three master builders, but now they won't actually pick up the gong piece. I've tried pausing the game and putting all three of them on the crate (so it says retrieving gong piece in their activity description), but after a little work they all wander off to do something else. How can I get them to pick up the gong piece?

Quit the game, relaunch it and try again

After they've opened the crate, you have to drop one of your villagers on the revealed piece to pick it up.

Still odd though.. cos he says the status line actually says retrieving gong piece.. be interesting to see if plopping other villagers down at the same time is affecting the first one picking it up...

Anything's possible. I know that putting more than one villager on the same task often messes things up, and nobody gets it done. Once the crate is open, it takes only one villager to get the piece.