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.
  For details, see "How do I install the bobbin wound with thread?".
   
• The bobbin is scratched or does not rotate smoothly.
  Replace the bobbin.
   
• The bobbin case is damaged.
  Replace the bobbin case with a new one.
  Consult your nearest authorized Brother dealer.


• The thread is tangled.
  Remove the thread jammed and clean the hook.
  For details, see  "Maintenance (Clean the hook)".
   
• A bobbin designed specifically for this machine is not used.
  Use the correct bobbin.
  
  The bobbin thread is not wound correctly.
  Check that the bobbin is wound so that it is about 80% full and that the thread is evenly wound. If the bobbin is not correctly wound, replace the bobbin with one that is correctly wound or rewind the bobbin.
  
  The lower thread tension is incorrect. The thread does not roll out of the bobbin case smoothly.
  Adjust the lower thread tension.
  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) ".
  
  The thread is loose in the hook area.
  Remove the loose thread. If the thread is tangled in the hook, clean the hook.
  For details, see "Maintenance (Clean the hook)".
  
  The hook does not rotate smoothly. Remove lint, clean and apply oil.
  For details, see  "Maintenance (Clean the hook)" , "Maintenance (Oiling the machine)".