If you were denied there should have been a "reason".. Of you Google "NALC mutual transfer denied" it will give you a few PDF files pointing you in the right direction. There was one branch that filed on it. But it is grievable, so best to contact the union. The language is in the JCAM. Hope that somewhat helps or get things going.
Yes we had a carrier, terrible sick leave record! He was denied a swap with another carrier. But the other Postmaster lied about receiving documents and made some mistakes on the transfer paperwork so we were able to take advantage of that and we got his swap approved. Usually easy to take advantage of management's mistakes