Are the notes overlapping?
In VOCALOID3, notes on the same track that are overlapping will not play.
To adjust overlapping notes, either use the mouse (left-click) to move them, or select "Normalize Notes" in the "Job" menu.
Also note that the positions in which notes are displayed and their actual positions might differ slightly, depending on the settings for "Quantize" and "Note Length".
If notes are displayed lightly even if it appears that they are not overlapping, either change the "Quantize" or "Note Length" settings, or select "Normalize Notes" in the "Job" menu.