If you can set areasa A & B with the engineers code it suggests to me that the system is programmed correctly. I'm thinking the issue is with the code you are using. Try setting up another user code for area B only.
Is Area B assigned to the user code?
I think you are presuming that both the A & B keys will be highlighted when you enter your code. This is not always the case.
Enter your code and the A key is highlighted. If you press the B key is that highlighted?
I know this is mainly a young mans game but jeez 28 mature engineer?
It's refreshing to read that the OP is 37 and still prepared to have a go at something new. Best of luck to you mate.