Hi,
I guess the first thing to do is to make sure the keyboard is properly seated in its holder. I would suggest removing it, check for any lint, dirt, etc. and then reinsert carefully making sure it is engaged in the connector first before pushing it down. This can be a bit tricky. You may want to try both of the possible orientations to be sure it’s a problem in both positions.
If you are absolutely sure that it is fully seated properly and still doesn’t respond, then see the the video here: https://www.youtube.com/watch?v=YR2RgUSTrss
It hows how to remove the top cover, once you get that removed you can see the cables from the secondary keyboard and where they attach to the PCBs. Make sure they are snug.
I’ve removed the top cover for repairs on two of my BS9000s, one for a loose keyboard cable and once to clean the gear drive. In both cases it was not too difficult if you have a video or the service manual. I recommend upgrading your membership to get access to the service manual, it’s well worth it.
Bill