There could be several reasons why the upper thread keeps breaking.
Check the following points or issues.

• The thread tension is too tight.
  Adjust the tension according to the type of fabric and thread being used.
  Polyester thread will stretch during sewing especially if the tensions are set too high.
  After the stitching is complete, the thread returns to its original strength, causing puckers in the fabric.
   
• Needle is turned, bent or the point is dull.
  Replace the needle.
   
• The framing tension of the fabric is incorrect.
  Tightly frame non-stretchable, woven fabrics.
  Loose framing will cause the fabric to bunch up under the stitching.
  Tautly frame soft knits using a stable backing.
  Overstretching the garment will cause it to look puckered when the frame is removed.
   
• The column stitches are too long.
  Re-digitize the pattern with fill stitching or with multiple rows of column stitching.
   
• The pattern density is too heavy.
  Too many stitches in an area pull fabric, causing it to pucker.
   
• There is a mistake in the upper or bobbin threading.
  Check the steps for threading the machine and rethread it correctly.
   
• Spool cap is set incorrectly.
  Check the method for attaching the spool cap, then reattach the spool cap.