Lots of reasons the thread may be breaking as mentioned a bur in the bobbin or catching the hook point when wrapping or too much pressure. Since I guess your using 210 Flat Waxed I would first look at the bobbin. I went through a lot of cheap bobbins before I just moved up in price and got better bobbins. Some bobbins that have the little insert on the tip and are still low cost, then insert may come unglued and then it's a real pain to fix so again a little higher priced bobbins will cure any trouble they may be giving you. Also if it comes down to your heavy handed and just breaking it, maybe you break down ads get a Rite Bobbin that has pressure control so you can over pressure it.

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