Check the following points to resolve the issue:
 

• Check the machine's environment. Conditions such as low humidity and low temperatures may cause this print quality issue.
  Refer to Product Safety Guide available on the Manuals section.
• Select the appropriate media type in the printer driver.
• Make sure the two green levers inside the back cover are in the up position. Pull up the two green levers until they stop.
  
• Clean the drum unit.
  > Click here to see how to clean the drum unit.
• Replace the drum unit with a new drum unit.
  > Click here to see how to replace the drum unit.
• Replace the toner cartridge with a new toner cartridge.
  > Click here to see how to replace the toner cartridge.
• The fuser unit may be contaminated. Contact Brother Customer Service from "Contact Us."