This happened in my game recently. Since the welcome wagon event did not trigger I was able to travel to another lot. When I got back the visitors had gone. A day or two later the proper welcome wagon event did happen and all was well.
Probably a delay between interactions, their next action might have been to come in and interact with something or someone but for whatever reason something is preventing the next action from triggering. i get that a few times when someone stands in front of the kitchen sink doing nothing and they finally go about doing something else later on.
