I've noticed that when my population gets to 26-27, and it's time to build the second hut, a villager (often one who is not a builder) will start to build it. The scaffolding will appear, but dropping villagers on it does not cause them to start building. Nothing appears in the status bar, either suggesting that they can build there, or showing the percentage of work completed. Instead, I have to wait until another villager wanders up and restarts the building work. After a little while I can then add builders as normal.

Anybody else noticed this? The construction of the third hut seems to go as usual.