This sounds as though the rubber contact is not working correctly on that key. My KN901 suffers from this and I have to turn off touch sensitivity so that all keys trigger at the same volume. I don't play the KN901 per se these days, I just use it for testing converted styles for my websites.

I've replaced the contacts several times on my KN901 (I have owned it a long time and used it in some very bad environments whilst playing for a circus). I could never repair the contacts by cleaning but I have seen some Youtube videos about how to clean them. I'm doubtful as to whether they can be cleaned effectively. You can't get new contacts as far as I know but they are usually reasonably easy to get from someone who is breaking a keyboard for spares.

Anyway, there's definitely something wrong with that KN2600. It is probably fairly easy to fix but the defect should be reflected in the price. KN2600 is a great keyboard, especially though a nice amplification system.

Cheers,
Mike