I have a consistent problem that on whatever piece I am working on, and whichever instrument, The topmost line is mute on playback. In the present case, if I put notes in the second violin part (same garritan instrument) I hear them, but in the first violin part I do not hear them.
When I enter them into the first part, I do not hear them then, either.
If I enter them in the second part (and hear them entering) and move them to the first part, I do not hear them. once moved. If I enter notes into the first part (and do not hear them when I enter them) and then move them to the second part - I do hear them on playback.
It happens with flute parts, too, if that is what I have on top of the score.