I have similar issues with my BA II plus Professional calculator. I dislike it a lot since the beginning. Keys feel too hard, I feel uncomfortable using it. The worst is; I often get wrong results because the keys won’t respond properly. Especially zero, like you mention,
And the last trick it pulled on me; two days ago, just days before the exam, all of a sudden my (+/-) key stopped working. I panicked. After a couple of hours it still didn’t work, so I quickly ordered another one. Some hours later, my husband came from work. He tried the calculator and surprise! the key worked perfectly. It still does. However I can’t trust it anymore…
I haven’t opened the new calculator yet, but maybe this one will turn out better; with softer, more responsive keys.
I suspect my first one is faulty because so many people seem to like this calculator.