There could be several reasons why stitches are skipped.
Check the following points or issues.

• The upper thread is not threaded correctly.
  Thread the upper thread correctly.

  For details, see "How do I thread the machine? (Threading the upper thread)" .
   

• The needle is bent or blunt.
  Replace the needle with a new one.
   
• The needle is not installed correctly.
  Correctly install the needle.
   
• Dust has accumulated under the needle plate or in the hook.
  Clean the hook and the needle plate.

  For details, see
  "Maintenance (Clean the hook) " 
  "Maintenance (Cleaning around the needle plate) " .
   

• The needle and the hook are not correctly passing each other.
  Consult your nearest authorized Brother dealer.
   
• The thread twist is either too tight or too loose.
  Improper twisting results in irregular loop formation. Try using a new spool.