I haven't watched loads of seasons @EachandEveryone. Obviously if one of then write leave and one of them writes stay at the commitment ceremony, they have to stay and try for another week - so in that instance, you might have someone who is adamant that it's not working for them, being pressured to keep trying (as in Luke and Melf)
Last year one of the contestants 'returned' with a new partner (Elliot). His old partner was still in it but there didn't stand a chance so they re-coupled him (with another lunatic unfortunately).
I have also seen someone leave because the experts realised they were a bully and still seeing their ex so got rid. In that instance his partner had to leave as well. They didn't find her a new match.