Some plates get hot in microwaves, and shouldn't be used in them. Other plates don't absorb microwaves and stay cool.

The difference is mainly in the type of clay used to make the ceramic plates (there's also some types of plastic that shouldn't be used in microwaves, but that's another topic). Clays that contain a lot of iron oxide, which tends to make for a dark reddish-brown colour though that may be covered by glaze, are especially bad for microwaving. Plates made from such clay can get very hot, and in the process they absorb a lot of the microwave energy instead of letting it go into the food. Hence, the food doesn't warm up as much as you expect.

Microwaves sort of bounce around inside the oven, reflecting off the metal walls, until they hit something that absorb them. That something then gets heated up. Glass and some types of ceramic (porcelain) are transparent to microwaves, just like glass is transparent to visible light. So if the microwave hits such a plate, it just goes right through and bounces off the oven floor. That wave then has a good chance to hit the food.

Plates made from red clay with lots of iron oxide, or that are glazed with some color made with a lot of metal or metal oxides, are opaque to microwaves and absorb them much like black paint is opaque to visual light and absorbs it. If a microwave hits such a plate, it gets absorbed and heats the plate instead of having a chance to heat the food.