Just like in baking pizzas from scratch, latent heat is the name of the game in reheating pizza slices. If the steel plate is too thin, it won't hold much latent heat, so it will cool more rapidly than a thicker steel plate or thicker stone deck. You also need to make sure the deck has been thoroughly heated, which might take an hour or more before reheating the slices on it. If you are reheating your slices in a home oven, try using a low rack position in the oven rather than a high(er) rack position. This will bring the deck closer to the heat source thus improving the bottom heat while reducing the top heat to the slice to help prevent melting of the cheese.
Tom Lehmann/The Dough Doctor