You might have been using too much. We also see this problem with commercial pizza ovens (deck types). During baking the veggies release water and you end up with a "swamp pizza". The air impingement ovens commonly used in many pizzerias direct a large volume of air onto the top of the pizza and in doing so exhibit excellent moisture management characteristics (sort of like passing a hair dryer over the pizza during baking). When you don't have much airflow over the top of the pizza it is difficult to manage this moisture. you might try finishing the pizza in a higher rack position so the pizza is exposed to more top heat to better evaporate the excess moisture. Blanching the veggies also helps, but destroys a lot of the texture at the same time. You might also look at how you are slicing the veggies too. Larger pieces are not as prone to moisture loss during baking as smaller pieces are.
Tom Lehmann/The Dough Doctor