Jeff is "spot on". One trick that I occasionally use is to place the tomatoes on a few pieces of paper towel to blot up some of the excess moisture/juice from the tomatoes. Also, make sure you're not overloading the pizza with tomatoes and/or fresh vegetable toppings. This can be especially problematic in a commercial deck oven or home type ovens with limited airflow characteristics. The air impingement ovens that are all the rage to day with the big box chains actually do an excellent job of managing all that moisture, but same cannot be said for deck, woodburning, or home type ovens. One thing that you might try is to very lightly paint the pizza skin with oil before you dress it. The oil will help to create a barrier to the penetration of moisture into the dough, thus allowing more of the moisture to evaporate without soaking into the dough during baking.
Tom Lehmann/The Dough Doctor