[Yeah well, She watches Inspector Space-Time so maybe they should totally geek out about BBC Sci-fi shenanigans together.]

Hmm... well, part of the reason people get sent back from what I can assertain is that we still can get sick while here; sick enough that even touch can't cure it. Perhaps sometimes they realise the person is incompatible from one universe, but not from another due to minor changes in that world? And other times, the person is drawn back from the original universe after some time has passed because they're more stable now.

Of course, this is all speculation...