The bobbin thread cannot be pulled up.
Needle is turned, bent or the point is dull.
Replace the needle.
Refer to How do I change the needle ?
Bobbin is set incorrectly.
Reset the bobbin thread correctly.
Refer to How do I set the bobbin ?
Upper threading is incorrect.
Check the steps for threading the machine and rethread the machine correctly.
Refer to How do I thread the machine ?
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