Hrm probably nothing RNG can be a PITA if you know what I mean.

Keep looking and make a point of reading all Lorestones you find some of them seem to be focus points for Boar spawns.

Just in case its a mechanical error I assume you have verified the game files with steam and there no mods involved her as some of them can trigger weird side effects - though not heard of this being one.

Edit: Oh one last thing occurred a second after I hit post. Make sure you keep killing the other stuff like necks so you make the game cycle creature spawns in the area your at. Might help can't hurt