Please check the following points:

• Shake the toner cartridges gently from side to side.
  
  
• Clean the laser scanner windows with a soft cloth.
  Please refer to the instructions, "How do I clean the laser scanner windows?".
  
  
• One of the toner cartridges may be damaged. Identify the missing color and put in a new toner cartridge.
  Please refer to the instructions, "How do I replace the toner cartridge?".