Hi S.F.Sorrow,
If somebody doesn't hear something, it's not there for them. IMHO, that's what your running into.
Try showing someone the difference between just and equal temperament when tuning a guitar, for example. Tune the instrument so the open A chord is just, and strum the chord some. Then strum an open D chord. Almost everybody with some rudimentary musical training can tell that the D chord sounds out of tune. Go back and play the open A chord some more. Then, tune the guitar E-A-D-G-B-E equal temperament and strum the open A chord again. After recently hearing the open A chord tuned justly, about half of your test subjects will be able to tell that the chord now doesn't sound the same, i.e., that it's not as resonant and satisfying in equal temperament. And yet, half of your test subjects won't hear any difference. The test subjects who don't hear the difference will not agree that there is any difference, often adamantly.
It stands to reason that certain resonances that can occur on a stringed instrument can and will tend to cancel first harmonics, etc., on certain notes. My Fender American Standard five string Precision has a typical Fender dead spot on the G string, 7th fret, and to some extent, half a step either up or down. The rest of the G string rings out with great sustain. On the 6th string, however, when playing a "D" the entire tonality of the note quickly diminishes to the point where that note sounds like it was played purposely staccato. Compared to the rest of the G string, the timber on the 6th, 7th and 8th frets is completely different (it sounds hard by comparison / no first harmonic) and is thoroughly devoid of sustain. Thud. Yes, on eighth notes and quarter notes the deal spot is less noticeable. But it's still there. Half notes? Nope. I'll play a half note on the 12th fret.
Some people say they can't hear the difference? Okay. Fine. I can't argue with that. Some of the more sophisticated digital audio FFT spectrum analyzers (not third octave) will show you the overtone series from a plucked or picked note on a stringed instrument. Or lack of same.