Those are the first 8 strips from the back of the cube. So counting on your last report, I would replace the strip immediately after J8, that is, J9. If the J9 strip is damaged then none of the strips that follow it (J10 onwards) will light up because of that one.
Or maybe the strip is not faulty; just not properly seated like the rest. Best way to make sure is to keep the cube running the demos (or any viz that was last loaded) and with the acrylic panels removed, gently and slowly wiggle the strip sideways, pressing down on its socket (without excess force), while observing it and the strips that follow.