Banking account - Fixed deposit

My father had a fixed deposit in a government bank of amount 10 lakh. The first name on the FD was his mom(my grandmother) and his was second name. My father used to keep that money aside to take care of my grandmom to avoid family quarrels related to money, so that my grandmom could feel secure thinking she has control on that. But my grandmom passed away last year and unfortunately my father also passed away in 6 months. There was no nominee on the FD. The FD's were on joint name of my grandmom and my father but her name was first. We have submitted the death certificates for both of those to the bank claiming that my mom is the only claimant. My father didn't had any brother or sister but bank is holding us up saying submit documents from other claimants of grandmom. What can we do ? Is there anyway we could provide a proof that my father was the only son. Its hard for my mom as she is senior citizen and has health problems, She needs the money to bear the medical expenses.