What Should I Do If the Bank Won't Honor My Power of Attorney?

My father and mother executed a co-durable power of attorney naming my brother and me as joint attorneys-in-fact. My father is now deceased. My mother is owner of an investment portfolio with monthly dividends being automatically reinvested. My brother and I wish to have the dividends placed into my mother's checking account so that I may utilize the funds for monthly fees at a 'memory-care center.' A large investment firm/bank doing business in our state of Washington will not honor the 'power of attorney' because it is a 'joint' power of attorney. Apparently the bank would honor the power of attorney if the document specified that my brother or I were able to act alone. My mother can no longer sign her name in order to modify the existing power of attorney. What legal grounds does a bank have to deny us using a power of attorney?