Did you set a long "Waiting Time"?

The "VOCALOID4 Inst" requires a waiting time before playback starts in order to process consonant synthesis.
Therefore, notes that fall within the range of the waiting time might not be voiced.

The same error might occur if the note falls within the range of the waiting time from the export position during audio mixdown, and within the range of the waiting time from the start of a part when freezing.
In such cases, set lower values for both "Waiting Time" and "Pre-Send Time" in the "Preferences" settings.

Consonant synthesis also requires a pre-send time.
A longer pre-send time will produce clearer consonants.
For the recommended settings, see the default values in the user manual.