You're using the lot challenge, right? There's a few things that can prevent an invasion from happening - vampires will only invade if everyone in the household is asleep at 12:00AM, and they won't invade if a household member is: a vampire, a ghost, pregnant, still affected by the vampire resistance drink or the supernatural shield, or has any of the "was recently bitten by a vampire" buffs.
The knocking on the door thing is a separate event that runs independently from the vampire invasions, so that shouldn't mean anything.