The title is really misleading. But nonetheless, they may not move in if you invite more than one villager to your town at once. If you save and continue after picking up the villager, and then the wifi disconnects, you will not get the villager. It may take up to 2 days for them to set their plot down.
Did you ask another villager to move in after you asked Marshal? If not, you may need to TT a couple days until his plot shows up. If you asked villager X to move in after asking Marshal to move in, and Marshal did not set up his plot yet, it may cancel Marshal out.