It's bit difficult to spot the issue. Here is something I found online. I think it will be useful to identify the issues. https://support.brother.com/g/b/faqend.aspx?c=us&lang=en&prod=hf_600_2eus&faqid=faqh00000318_000#:~:text=Make sure the spool of,for the size of spool.&text=The bobbin thread is not,correctly wound%2C rewind the bobbin.