I suppose this is some polyphonic audio material? And is it Melodyne editor that you are using? Or Melodyne essential, as your profile says? Are you aware that you need the DNA technology in Melodyne editor for accessing and editing individual notes in chords and to avoid artefacts?
Also, for best results it is crucial that Melodyne has detected your audio correctly, e.g. that it finds all the notes properly and that all note separations are at the right place. Please see this tour
from our online manual for details.