Use the model's cooking instructions. A grill's preheating period can expose food to a different early heat pattern from the intended cooking stage.
What the observation means
The pan, grid and door arrangement also matter. A generic recipe may assume that the grill is ready before the food is placed beneath it. Delicate toppings can scorch quickly during an unsuitable preheat.
A useful next step
Follow both appliance and food guidance. Record whether food was present from the beginning if the result changed. Do not leave the grill unattended, and do not extend preheating indefinitely to compensate for a suspected lack of heat.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.