How do I stop a bank from acknowledging a Revocation of DPOA issued after the principle was deemed incompetent?

My father has financial DPOA on my grandmother. The DPOA was sprung by two doctors' letters deeming her mentally incompetent (Alzheimer's). A bank account was opened in her name, utilizing the DPOA and doctors' letters. My grandmother has since signed a Revocation of DPOA and my uncle has sent it in to the bank. My grandmother doesn't understand the consequences of her actions and in fact doesn't recall signing the document a week after doing so. My uncle is attempting to gain access to my grandmother's money. The bank has stopped recognizing the doctors' letters they used to set-up the account and honoring the revocation. I don't understand how the bank can flip / flop on their position. If they bank would have not recognized the doctors' letters to begin with, the account would have been set-up at a different bank. Please note that two other major banks (Chase and BofA) have recognized the sprung DPOA as a valid instrument for my father to conduct financial matters on my grandmother's behalf. How do I get the bank to change their position without filing for conservatorship/guardianship?