The reason for this could be one of three things. The first is adding a very thick layer of sauce to the dough. If you have added a very thick layer to the dough, the liquid from the sauce can add extra moisture to the dough, making it wetter and can cause the dough to go soggy. We like to use about 2-2 ½ tablespoons of bottled pasta sauce for a 30 cm pizza base because it is quite a wet sauce. If you are using a thick tomato paste with very little moisture, you could get away with using a little more. The second is having the topping sit on the pizza dough for too long before baking. You may have added the toppings and had them on the dough for too long before baking. The best way to avoid this is to bake the pizza soon after adding the topping. The third is overloading the pizza with toppings. If you have put too many ingredients on the top, it will sit heavy on the dough and won't cook through.