Frank, I googled 'Saputo' and, from the packaging, it looks like low moisture brick mozzarella, which is what you want. Brick can vary, though. You might want to try different brands.
As Bill, mentioned, whole milk is critical. You're grating the cheese yourself, correct?
The other important aspect to properly melted cheese is crust thickness. In order for the cheese to bubble, the crust must be thin. The less dough you have underneath the cheese, the less insulation you have, the easier it is for the bottom heat to reach the cheese and bubble it.
Heat is also important. That intense, cheese bubbling bottom heat needs to come from a thick conductive stone or a modded oven.