Question about WMF Lumero Gourmet-Station 3-in-1

After the first raclette, I noticed that the left-hand control makes a little clicking noise and changes colour when the set temperature is reached, as with the oven (preheating). But in the last 1/4 this no longer happens. is this normal or can I not heat up to 100%? only up to approx. 80%? Defective controller? When I unplug the appliance, the clicking goes back continuously to 0%. I hope someone can help me.