JamieOliver.com

It might be the dish on which you are cooking the pizza. Have you ever placed a piece of hot toast on a plate? Moisture collects on the plate and toast. So when you place your pizza on a dish I guess the moisture has got no where to go as it can't "escape" so the pizza bread will absorb it. You can buy pizza trays which are circular dishes with holes punctured all over the tray. These holes, presumably, allow moisture to escape from the pizza bread. Now of course you can use a pizza stone and I don't know why that would prevent the bread going soggy. Try just place your pizza directly in the oven without a tray. Please note that I am speculating. 

I agree with the technique of putting the pizza onto a hot tray (if not using a pizza stone). Heat oven & tray until hot, and then slide pizza onto tray to cook. 

Pizza trays with holes in the bottom work great.
Also, drizzle a little oil and spread tomato puree on top of your pizza, but not too much! Wait until your pizza is almost cooked then add a little extra ready-made puree - if needed - and the rest of your topping.

Welcome to the forum jesswareing1  and domesticdeity   

jesswareing1 , it could be one of a few things...

as rasher says, pizzas need a realy hot oven , maybe you have not waited for the oven to heat enough before you start to cook the pizza.

With topping Pizza... definatly a case here for ...' Less is More ' , again as rasher says..
"Look at any proper pizza you get at a restaurant and you'll see that they don't pile on the toppings."
The sauce of the pizza in an Italian restaurant is spread quite thinly towards the edge of the pizza , but not up to the edge.
A scatter of cheese is what goes on next as this will melt onto the sauce and help any topping adhere to the pizza.
Next a few bits of topping... not much is realy needed , but if you are putting something like mushroom on the pizza I find that it is often best to quickly cook them  a little first( as mushrooms hold a lot of water).
Into a very hot oven and 

In the past I have cooked Pizza strait on the oven shelf , but they do go a bit wonky ! I never forund that to sucessfull and so I would place a large length of non-stick kitchen parchment (baking ) paper strait onto the oven shelf and cook the pizza direcley on that.

I am sold on my Pizza stone, this once heated will reduce the cooking time of the pizza by quite a lot and also get the base crisp
I had a friend that for years had a stack of thick quarry tiles that he would place on a metal tray and use instead of a pizza stone , he found that this worked mush the same.

I cured the over-moist tomato base like this;

http://www.jamieoliver.com/forum/viewtopic.php?id=36151

I guess you could do it without a pressure cooker, perhaps in a low oven.  I used to suffer from a soggy bottom particularly in Calzone until I used this.