Sorry to hear that you are having problems getting the password reset.
The quickest way to get the password reset would be by calling 0800 800 150 as we currently have a large number of cases in our queue and it's taking longer than 72 hours for us to pick up and work cases.
If the email account is a subaccount of your email account have you checked to see if it is showing in your MyBT > Manage Email.
If it is and assuming you have not "gifted" the email account and it now has its own MyBT, you should be able to change the password there.
If you are asked security questions it is the master account's answers that are needed.
If the email account has been "gifted" you could try entering its MyBT and change the password from there.