Some breakers have a screw that connects it to the pole bus! Other have a press connector.
Possibly, the screw wasn't tightened properly when it was installed. or it wasn't pressed into place properly.
As it was loose, damage may have occured where the breaker contacts the bus.
If you cannot tighten the mounting screw, you may be able to relocate the breaker to a spare position.
If you have no spare positions, try a new breaker.
REMEMBER TO TURN OFF THE MAINS BEFORE WORKING IN THE PANEL!
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