User blog comment:PurpleAmaryllis/Slightly worried/@comment-1519608-20120512044232

That generally happens with the default game Story Progression settings. The game's story progression decides to move Sims out to simulate real town activities. I had it really bad on one of my neighborhoods- the game moved away most of the families, even ones I had created.

I know that I, as well as a lot of other players, have downloaded Story Progression mods to address this problem. My personal favorite is Twallan's StoryProgression - it fixes bugs in the default system and makes the neighborhood Sims a lot more dynamic than they otherwise are. Here's a link to Twallan's mod. There are several others out there, running a simple web search can find some of them.