Please check the following points to solve the problem:

• Clean the corona wire inside the drum unit by sliding the green tab. Be sure to return the green tab to the home position.
  Please refer to the instructions, "Clean the corona wire."
  
  
• The drum unit may be damaged. Put in a new drum unit.
  Please refer to the instructions, "REPLACE DRUM".
  
  
• The fuser unit may be contaminated. Call Brother Customer Service.