There could be several reasons why the needle threader cannot be used.
Check the following points or issues.

• The needle is not installed correctly.
  Correctly install the needle.
   
• Needle is turned, bent or the point is dull.
  Replace the needle.
   
• Recommended needles are not used on this machine.
  Replace and use recommended needle.
   
• The hook on the automatic threading mechanism is bent.
  Consult your nearest authorized Brother dealer.