Please check the following points:

• Make sure you use paper that meets the recommended specifications.
  To see what kind of paper you can use, click here.
• Check the printer's environment - high temperatures and high humidity can increase the amount of background shading.
• The toner cartridge maybe damaged. Put in a new toner cartridge.
  To see how to replace the toner cartridge, click here.
• The drum unit maybe damager. Put in a new drum unit.
  To see how to replace the drum unit, click here.