In the very first seconds of a match, it often happens, that villager training commands are not being registered by the game.
That means:
Game starts, you immediately select the town center and click on “train villager”, for example 3 times.
Then move your starting vills and scout and sheep and select the towncenter again.
But there are no villagers in training queue.
This bug has been existing since betas, as far as I remember and happens quite frequently.