I am pretty sure the missed note has to do with your instrument skill
associated with the song your are playing. I remember when I first hit
level 16, I could play the level 16 without any problem even when I have 0
skill in wind instrument. Then a few days later, I couldn't get the song to
start without 10 or 15 missed note. Somehow Verant had changed the song
without telling us.

I think the same thing happened to the rest of the songs since the last
patch. If your instrument skill for that song is way lower than your
current level skill cap, that might have caused the missed notes. I hardly
ever get a missed note now, but all my instrument and singing skills are at
200. My dex is also over 120.. so I don't know if that could be the reason
or not.