I am having a similar issue with my rochelle that i got about a week ago. whenever i press down my f#/d button on the g row, the b/g button just above it comes down, as well. I opened the concertina up, to see what the matter was, but I couldn't find the problem:(
I would advise dropping Wim Wakker a line. He was quite helpful on my behalf and the response was right away. Others here are likely know how to fix your specific issue though so I would not be at all surprised to find many answers in short order. Good luck, Dave. aka up-fiddler