I have a small Janome 3000 and had so many problems with the bobbin case. The serviceman put in a larger case and it worked ok for some tiem. Then I had the problem again, took it in and it checked out ok. I had rethreaded it several times and reinstalled the bobbin numerous times too. The Janome service man noted the thread would wrap around the needle which caused the thread to break.
I also noted that cone shaped thread would wrap around the thread holder and make a mess. So I am trying to stay with evenly wrapped thread, and prefer YLI.
My next machine will be a Bernina830.