Check the following points to resolve the issue:

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

• Open the back cover and make sure that the two green envelope levers on the left and right hand sides are in the up position. Close the back cover.
  
• Select Improve Toner Fixing mode in the printer driver.
   

  For Windows users:
  NOTE: The steps and screens may differ depending on the operating system and application being used.

   
  1. From the menu bar of an application, click File => Print.
     
     
      
  2. Check your printer name is selected and click Properties.
     
     
      
  3. Click Advanced tab and then Other Print Options.
     
     
      
  4. Click Improve Print Output and select Improve Toner Fixing.
     
     
      
  5. Click OK to close the Other Print Options dialogue.
  6. Click OK to close the Properties dialogue.
  7. Click OK. The document will be printed.
     If this selection does not provide enough improvement, select Thicker Paper in Media Type settings.
   

  For Macintosh users:
  NOTE: The steps and screens may differ depending on the operating system being used.

   
  1. Open Print dialog. (Click here to see the details of how to open Print dialog.)
  2. Select Print Settings from the print options pop-up menu.
  3. Click Advanced and then Improve Print Output.... 
     
     
      
  4. Select Improve Toner Fixing.
     
     
      
  5. Click OK.
  6. Click Print.
     If this selection does not provide enough improvement, select Thicker Paper in Media Type settings.