Make sure the drum unit and toner cartridge assembly is installed properly. 
If not, reinstall the drum unit and toner cartridge assembly to the machine as follows:

NOTE: Illustrations shown below are from representative product and may differ from your Brother machine.

1. Open the front cover.
   
   
   

   

   
   
2. Take out the drum unit and toner cartridge assembly.
   
   
   
   
   
   
   • While removing the drum unit, handle it carefully because it may contain toner.
   • We recommend that you place the drum unit and/or the toner cartridge on a clean, flat, level, stable surface with a sheet of disposable paper or cloth underneath it in case you accidentally spill or scatter toner.
   • Handle the toner cartridge carefully. If toner scatters on your hands or clothes, immediately wipe or wash it off with cold water.
   • To prevent damage to the machine from static electricity, DO NOT touch the electrodes shown in the illustration.
     
     
     
     
   • To avoid print quality problems, DO NOT touch the shaded parts shown in the illustrations.
     
     
   
   
3. Put the drum unit and toner cartridge assembly back into the machine.
   
   
4. Close the front cover.