Yamaha CLP-635 Highest six notes sound dead, almost no tone

On my Yamaha CLP-635 I just recently noticed this issue because previously I never used these highest six notes but now I’m learning Flight Of The Bumblebee and the last section requires these notes. There is no ring whatsoever to these notes, just a “dink” and the thud of my fingers is all I hear, I can’t even decipher which note is which. Changing to a more sensitive touch seems to help a little but not enough. Is there a way that I can make these highest notes ring out more?