I have been having the EXACT same problem. I am so very frustrated. I have had my machine for 6 months now. It has done it off and on. I have always had to take the plate off and take the bobbin casing out, etc. I did find that the needle had cut a bur in the bobbin casing. I filed it down and it began working again. But then it would always eventually happen again. I could usually keep on and on and finese it out of its fit. But a few days ago it began to do it constantly.
I took it to my authorized Brother repair man. He replaced the bobbin casing and said it sewed beautifully for him. I was so happy and brought it home. The VERY FIRST STITCH for me it did the exact same safety lockdown again! I called basically in tears and he said to bring it back in again. I dropped it off this morning and was called and told it was ready to go. When I picked it up this afternoon, AGAIN they showed what they stitched out and said that it had never caused a single problem for them.
I was skeptical but happy and came home to try it out again. It immediately started happening to me AGAIN! I forgot to mention that this is my 2nd Brother embroidery only machine and I have been embroidering for over 6 years with no problems until this machine. My other machine was a PE-150. I feel like I am going crazy! How can it stitch perfectly for him and do NOTHING for me??
Thank goodness, I have had one friend over watching me embroider something for her and saw how it was fine for most of her item and then went into the lockdown mode, OR else I might think I need to have my head examined. Needless to say, I now have a (in my opinion) expensive machine that will not work AT ALL for me and the repair man said he can no longer help me with this problem at all. He said I would just need to mail my machine to a Brother repair facility.
I am using the correct bobbin, I am using Brother brand bobbin thread, I have used numerous different top threads, I have reduced the top tension, I have used many different items and stabilizers, ALL with the same results - LOCKDOWN!
I am ready to throw this machine in a dumpster and cry! I truly LOVE to embroider things and am beyond sad about this whole situation :(.