when you switch a bank, select a tone while the bank numeral is flashing or the bank will switch back to the prior bank.
Sounds simple, but my assumption was that all you had to do was to press A&B or C&D and the bank would change...and then you would select a tone - one thing at a time. Actually, I still think that's how it should work, but now at least I know.
After 5 videos and the quick start guide, when I still couldn't make it change, I assumed it was a bug and returned it. After the 2nd one, I watched a few more videos and found what I was looking for.
A real manual would have helped.