Most adult musicians (and many student musicians) know exactly what they need to do in order to get the best sound out of their instruments while moving from one note to the next. We teach techniques to our students, and we practice them when we play our own scales and etudes.
We strive to pay attention at all times so that the things we need to do in order to sound good all the time become nearly unconscious. There is a point, however, that those bits of technical know-how become so unconscious that we no longer give them the attention they still need.
The act of playing well is the act of constant attention. If we don't pay attention to the way we sound in passages that are naturally less resonant (we all come across pesky passages that are difficult to play because of physical and/or harmonic reasons), the less-than-ideal sound we make becomes a sound we accept and sometimes even ignore. It's kind of like the dust that accumulates in corners, on Venetian blinds, and on the tops of books. You clean them once, and somehow, while we are not looking, the dust returns.