I now often have this problem. However a day later the Sim in question might show up.
FYI this issue seems to have been fixed for me somehow. I can't point to any specific change I made on my part... but it cleared up after the last special event ended.
For me it really depends on the Sim. It's mostly other players Sims that don't show up for me.
I've started to experience this, too, but only with one particular sim that doesn't come; this sim is controlled by another player.
July 2018 - last edited July 2018
I still have this bug. It resolves itself, and luckily I've been able to find the Sim I'm trying to build a friendship with.
The one in pink is always hanging around Downtown. She will stand there until you talk to her. Talking to her won't make her show up when you call her over though.
(she is still standing there, even after finishing this message)
Also I noticed that Sims at my party have this standing around problem as well
And at Market Square, nobody is showing up at all.
This made me wonder if it's all related @SimGuruMaaike.
I also had this issue some months ago but not anymore. Sims show up at my sims home immediately in order to socialize.
The other two issues you mention, @Phantomlover1717, are still the same. Sims stand outside when I'm hosting a party. Is it supposed to be like this?
Market square is a ghost town.