Not sure this would work with all plates, unless it's an available accessory.
Can you post links to the thermostat and plate that you use it with @BiggDeal ?
If the chicks haven't used a plate before, you might have to 'show' them by holding them under until they feel the heat. Turn off the heat lamp and make sure the plate is at the right height for them to touch their backs to it. Plate should be a tad higher in front than in back so they can find...