Oven temperature and rack levels
I enjoy TJ's frozen pizzas. Many of them require an oven at 425 degrees. When I set my oven to that temperature and place the pizza on the middle rack, the bottom burns while the toppings are not quite done. So now, I heat the oven to only 400 degrees and put the pizza on the top rack. It's a little better, but the cheese turns out dry, rather than gooey and moist.
Any suggestions? Thanks in advance
Place something between the heating element and the food being cooked. You haven't noticed this before because you usually cook using a vessel (pot/pan/etc.)
One way to deal with this it to use a pizza stone or tiles, or, you can put a cookie sheet on the lower rack, aluminum foil will work but not very well, you want something the will deflect the direct heat from the crust. Maybe this is an analogy. If you sit on a lit stove burner, you will burn your bottom, if you put a heavy pan on the burner and sit on that, you will warm your bottom (until the pan gets hotter).
My oven can has 3 or 4 rack levels, I put the bottom rack on the lowest level, and a pizza stone on that, the other rack is one from the highest level. My pizza stone is somewhat smaller than the bottom rack, with the space at the back of the oven. My feeling is, if the pizza stone is too large, it will trap too much heat below it. If you do not want to purchase a pizza stone, many people are using "tiles". I don't know the exact name of the tiles, they may be called quary tiles. A google search will reveal what type of tiles to purchase from a home improvement store.
In addition, do not put the pizza in the oven until it is completely pre heated. Once an oven is pre heated, the heating element will "rest".
Do you have an electric or gas oven?
Don't want to make a dumb suggestion, but is it possible that the electric element or gas flow from the top of your oven is not working? If that is the case, it would explain why the cheese is heated but not melted.
It is also possible that your oven temp is off. From what I know, many (even most!) ovens are off. Get a thermometer and check: when you set it to 425, what is the real temp? Even professional ovens go off.
If you just have to cope, you might consider cooking the pizza till the crust is to your liking and then broiling the cheese a little to melt and brown... that is, if the top element/gas flow for the broiler works okay.