Check the following points or issues.

• The upper thread tension is low.
  After passing the upper thread through the hole in the presser foot, pull the thread by hand to check the thread tension.
   
• The thread tension does not correspond to the amount that the thread tension knob or the upper thread guide pretension knob was tightened.
  If the thread tension cannot be adjusted, dust may have collected in the upper or middle thread guide plates, causing the thread tension guide plates not to measure the thread weight properly.
  Clean the thread tension guide plates.
   
• Thread is not correctly threaded around upper thread tension discs.
  Clean upper thread tension disc. Rethread and make sure tension disc rotates when pulling on thread.
   
• The thread quality is poor.
  Try sewing with different thread.
  If the problem no longer occurs after the thread is changed, the thread quality is the problem. Replace the thread with one of good quality.
   
• The lower thread tension is incorrect.
  Adjust the tension of the bobbin thread.