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.

  For details, see "How do I adjust the tension of the bobbin thread ? " .
  
   

• Lint or dust has accumulated in the thread tension disc. 
  Remove the tension dials, and then remove any lint or dust accumulated between the felt washers.

  For details, see "Maintenance (Cleaning the thread paths of the upper threads) " .