Hi, are you sure because maybe they do when you don't see it, once in 24 hours is enough did you check his belly? I don't know the right answer as only helped feeding babies with their mother's milk sometimes one or two are weaker than others but never actually had situation when mother and foster stopped feeding completely. Is there any reason for that maybe you can try to feed him/her bringing to mother like here in this video? I believe once a day would be enough but if weaker you can do twice if mothers have milk.
Hope your situation will change for good soon please keep us updated