So I got as far as trying this
This looks like one place where the code is setting up two 16-element arrays. Perhaps change '32' to '40' and '16' to '20'.
Look for other places in your sketch where the constants 16, 15, or 32 are used.
This seems to now work as far using all 20 columns, a sweep with my waveform generator shows each peak reacts well. I am still trying to make sense of what the other posters have mentioned which I am sure will wind up fixing the peak height.