Please check the following points:

• Check that the recommended paper was used.
  To see what is the recommended paper, click here.
  
  
• Check that the back cover is closed properly.
  
  
• Shake the toner cartridges gently.
  
  
• Clean the laser glass.
  To see how to clean the inside of the machine, click here.
  
  
• Turn off Toner Save.
  To see how to turn off Toner Save, click here.
  
  
• Change the paper to freshly unpacked paper, and then check that the problem is solved.