Start with room temperature Mozzarella.
The toast coming out of the oven will be hot and will start to melt the cheese.
If your cheese does not melt place lid over the skillet and remove from heat. Let the sandwich sit in the skillet for a few minutes to allow the cheese to melt.
Because mozzarella slices can vary in thickness and if the method above is not working place in the microwave for 30 seconds.