Please check the following points:

• Identify the missing colour and put in a new toner cartridge.
  To see how to replace the toner cartridge, click here.
   
• Clean all four corona wires (one for each colour) inside the drum unit by sliding the green tab.
  To see how to clean the corona wire, click here.
   
• Make sure the corona wire cleaners are at their home positions.
   
• Put in a new drum unit.
  To see how to replace the drum unit, click here.