This is one of those problems that you think you've solved, only to discover that the problem still exists. The best workaround we can think of: overdoing due diligence, well in advance.
When you took the necessary and advisable step to have a parent or other relative sign a durable power of attorney, you might have felt relieved, secure in the knowledge that you were now prepared and protected to handle their finances if they become incapacitated.
However, The New York Times explains in “Finding Out Your Power of Attorney Is Powerless” that when you take the witnessed and notarized document to a financial institution, the officials there may not accept the documents. They may not honor your power of attorney and may insist that the account owners sign the institution’s own power of attorney form.