Please check the following points:

• Clean the laser glass.
  To see how to clean the inside of the machine, click here.
  
  
• Identify the missing color and replace that toner cartridge.
  To see how to replace the toner cartridge, click here.
  
  
• The toner has probably reached the end of its life. However, when you see a white vertical line on the toner roller, try the following procedure.
  
  
  1. Cut a sheet approx. 1.97 in. x 1.97 in. (50 mm x 50 mm) out of a transparency film.
  2. Insert the film about 2.54 in. (10 mm) into the gap between the toner roller and the blade.
  3. Slide the film and pull it out as shown below.