There could be several reasons why the tension is incorrect.
Check the following points or issues.

• The upper thread is not threaded correctly.
  Check the thread path, and then thread the upper thread again.

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

  
   
• The bobbin thread is not correctly threaded.
  Correctly thread the bobbin thread.

  For details, see "How do I install the bobbin wound with thread? " .

  
   
• The lower thread tension is incorrect.
  Adjust the tension of the bobbin thread.
   
• Bobbin thread is incorrectly wound.
  Use a correctly wound bobbin.
   
• Needle is turned, bent or the point is dull.
  Replace the needle.