Trapped sims disappearing ?

I have an evil sim who kidnaps other sims and locks them up in her basement.

Once the sims are in their cell I delete the door so there is no way they can leave ... yet every so often one will simply disappear.
The first three she 'caught' are still there , but the fourth and sixth sims just up and vanished (she has eight trapped down there at the moment)

So how come some can get out and others cant ?