your oven might be calibrated wrong. a lot of ovens are, that's why you see a lot of chefs using external thermometers that have a probe in the oven to get accurate readings. you might be putting it in a cold oven when the package says to preheat it. or you might be putting the pizza on the top rack. hell, you might not even have the same definition of "done" as tombstone or digiorno. point is, temp x time = done, and somewhere in that equation one of your variables is different than the pizza guys. find out what it is, and you'll be ok.