There could be several reasons why the bobbin thread breaks.
Check the following points or issues.

• The bobbin thread is not correctly threaded.
  Correctly thread the bobbin thread.
   
• The bobbin is scratched or does not rotate smoothly.
  Replace the bobbin.
   
• The thread is tangled.
  Remove the thread jammed and clean the hook.
   
• A bobbin designed specifically for this machine is not used.
  Use the correct bobbin.
   
• The bobbin case is damaged.
  Replace the bobbin case.