1. You need to remove the thickets (bones, grass etc) on your side till the boar appears.
2. You can`t get any thickets in your village if you have not enough free place.
3. The more days you miss in the game, the more thickets will appear on your territory.
4. To increase the chance to get the boar you should begin with removing the wild flowers and grass and only then remove the bones, stones etc.
5. The thickets do not grow next to man-built and other decoration items in the game.