Transfer of Flat after death of my Grandfather.

Hi, We had one property (Flat in Cooperative Housing society). This property was on my Grandfather's name. He had already made my My mother (his daughter in law) a nominee in Society's record & had also done a registered will which clearly mentions that this property should be transferred to my mom, me & my younger brother after his death. Now after clearing all dues of pending maintanence, we applied in society to transfer the property to us as per nominee and will. Society called for a meeting and on that one committee member is objecting saying we have not presented Share certificate in the file. As my grandfather was very old and use to stay alone in the house as per his wish and we use to stay in neighbouring society on rent taking care of his all meals of the day & other medical responsibilities. Last year before his death, there was a flood situation in our area where water was clogged in my grandfather's house too. During that time maybe his share certificate was lost. Can society object and stop transfer without share certificate? They already have a copy with them in their records than why this objection? Nomination in society and registered will are not enough? FYI - My father has already expired 5 years ago and thus not have anything related to him in this case. Please help & guide. In a very critical condition.