@Briana2425 Do you mean your Sims randomly disappear off screen (with or without a plumbob remaining to show you where they're supposed to be, and there's an empty space where they should be listed at the bottom of the screen, but they still show up in the family tree? If so, or if that sounds close, then I've had that as well and its a bug that is listed as being fixed.
https://answers.ea.com/t5/Bug-Reports/FIXED-Sims-in-Family-Tree-Disappearing/m-p/7287642/highlight/true#M80659
If this is still happening for you then report the bug here: https://answers.ea.com/t5/Bug-Reports/bd-p/The-Sims-4-Bugs