She may not be in the social security death index for a number of reasons; she may not have had a card if it wasnt required for a job; she may have had one, but died prior to the index in the early to
mid 1960's, or noone reported her death to SS
Have you checked for obits for any siblings or children? That might give a location or other married name for her. Was Downing her middle name, or her middle name? Do you have an exact birth date?