Boaz

That is likely it. The point of a heat plate is a gradient, and in a small brooder with the plate in a corner the whole box ends up warm. Bigger brooder or less heat, and check at chick level in several places so you know what you actually have.

Away from the heat rather than under it points at overheating rather than chilling, which is the opposite of what people assume when chicks die. Measure the temperature at chick level, not at the plate, and make sure there is a genuinely cooler zone they can get to.

Chilled chicks huddle under the heat and peep loudly. Overheated chicks spread out as far as they can get, pant, hold their wings away from the body and go lethargic. Yours were found at the far end.