I thought I was having the same issue. My new household, on a completely new save file, no mods or cc, moved into 17 Culpepper in San Myshuno, but the welcome wagon seemingly never came. I went through several in-game days of play and no welcome wagon. I saved the game and exited.
The next time I played the game, about 2 more in-game days passed when the welcome wagon event finally SEEMED to appear. However, the 3 neighborhood sims just stood at the door, fruitcake in hand, never actually knocking or initiating the event. They weren't behaving normally, either, because their idle animation kept resetting every 1-2 seconds, almost as if they were continually trying to queue or clear an action. They stood there so long that they started wetting themselves on the ground, and one of them nearly starved to death. I had to use the "Send Home" interaction (with the orange circle, the one that forces them to clear their queue and leave immediately) just to prevent them from standing there until they died. The next in-game day after that, they spawned and properly knocked on the door, initiating the event. Overall, the welcome wagon eventually did trigger, a whole in-game week after the household moved into the lot, and only after one bugged attempt.
I have no idea what made it bug out so badly. The only thing I can think of is that when the welcome wagon did attempt to trigger the first time, there was already a sort of "event" happening where one of my neighbors was doing that "loud neighbor" thing, with a bunch of noise coming from their unit. The neighbor was Raj Rasoya, and I believe he was supposed to be part of the welcome wagon with his mother, and since he was already involved in some other event happening, it broke the welcome wagon. He never appeared like he was supposed to. Because the next time the event triggered, he was there with the other 3, as expected.
I gave a vote on this because it did seem to be bugged, regardless of the fact that it did eventually trigger for me.