A damaged instrument can make a skilled player sound less capable than they are.

Performance is a partnership between a person and their equipment, and an audience only hears the result. When the tool is failing, the skill is still there and cannot come through: the note does not speak, the action sticks, and what everyone perceives is a player falling short. Judgement lands on the person because the person is the visible part.

Before concluding that someone is not good at something, check what they are working with. And if your own results have got worse, look at the conditions before looking at yourself. Blunt knives and bad chairs have ruined more work than lack of talent has.