buxar post office didnot refund my mothers money while i am in nominee

Sir My mother had an account in post office ,buxar,Bihar Account number -218858 In the account at least ₹42000 are there . Me and my two brothers are there in nominee .after our mother death we fill the nominee form in which my mothers death certificate copy are attested and send it to post office ,they also take the account paasbook from us .Now 6 years ago But there is no response given by post office to us Sir please help us