Yes, sometimes it just isn't a good fit. Sometimes you can meet the child with the parents even and they turn into a different child on the first day.

I like to wait a few weeks to see how well they adapt to my environment and adapt/listen to my rules. If they make progress, I hold on to them. If not... out they go.

If they aren't a good fit for you, then you are not a good fit for them and they will probably thrive in a different environment.