Have you been checking the signboards for multiple days in a row, and/or resetting the game before you've saved for the first time in a day?
If not, it's probably a matter of small sample size. August counts as winter in the in-game seasons, and there's a 1/17 chance that route 8 was selected as the swarm route for that day. However, the only wild encounters that can ever appear on route 8 are those from sploshing through shallow, marshy water (or surfing in the deeper water), and in winter all that water is frozen over, making it impossible to get encounters at all there. Hence, if route 8 is chosen as the swarm route but it's a winter day, the signboards will not report a swarm, and and all you can do is try again tomorrow, or reset the game to have it roll for a new route if you haven't saved yet today.