The bobbin thread breaks.
There could be several reasons why the bobbin thread breaks.
Check the following points or issues.
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The bobbin thread is not correctly threaded.
Correctly thread the bobbin thread.
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The bobbin is scratched or does not rotate smoothly.
Replace the bobbin.
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The thread is tangled.
Remove the thread jammed and clean the hook.
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A bobbin designed specifically for this machine is not used.
Use the correct bobbin.
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The bobbin case is damaged.
Replace the bobbin case.