FNB refusing to accept Power of Attorney

FNB is refusing to accept a legally binding signed power of attorney to gain access to a bank account. My father in law is bedridden in a state hospital, and have signed a power of attorney over to his wife of 42 years, married in community of property, which is certified and legally binding. They are refusing to give her access, stating that they will only accept a document signed by the master of the high court. I have tried contacting several people with little or no help, and no written document can be given where it states that the power of attorney is not valid, or valid reasons as to why the power of attorney will not be accepted.