Hi samusko and welcome to Talking Point.
Can you confirm whether the bank account is in Scotland or England? As far as I know the POA should apply ok in either country but I believe the banking rules are slightly different between the two.
In simple terms the POA should allow you to act as though you actually are Mrs A so if Mrs A can access the account I would have thought that you should be able to as her attorney. If there are any accounts only in the name of Mr A then neither you nor Mrs A will have access but that doesn't help for the joint account
Would it be feasible to take Mrs A to the bank (for one time only!!!!) to remove all/most of the money in the account and deposit it into an account only in her name and then get the bank to give you access to that account?
The possible problem with that approach depends on whether the local authority is now, or likely to be in the future, providing funding for Mr A's care. Removing money from an account where technically he owns 50% of it and putting it into a single name other than his could be classed as deprivation of assets.
Who of why do banks make life so difficult
Good luck with getting something sorted out!