Redistribute the toner in the imaging unit.
Remove the toner cartridge, and then the imaging unit.
Firmly shake the imaging unit.
Warning—Potential Damage: Do not expose the imaging unit to direct light for more than 10 minutes. Extended exposure to light may cause print quality problems.
Reinstall the imaging unit, and then the cartridge.
Resend the print job.
Do vertical voids appear on prints?
Contact customer support.
Note: The laser scanning unit may be defective.
The problem is solved.