As above, you've got the decapping pin set to high up in the die. You're trying to pull the ball through, while the neck is still in the narrow part of the die. You're also working the living hell out of the brass, and making it hard on yourself, and the press.
The other way, would be it you are setting it way too low, and it is hitting the bottom of the case, when you're decapping. That's more likely to result in a bent stem than a broken ball, but possible I guess.
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