If you have 8 villagers, the game will try to move in at least one new villager. The only time the game doesn't do that is if you have 9 (in that case, the only uninvited move-ins will come from other people's voids).
Do you have any lazies in your town? Where random move-ins are concerned, the game usually tries to move in a personality that you don't have yet. If you have one of each, it'll pick a random set of villagers, usually of the same personality. I think it's still possible to reset to have more control over which ones you get, some people have obtained dreamies that way.