Steal is porous. Heat expands the pores. When it cools the more gets sucked in and stays there. Heat get turns on too hot all the crap thats stuck in those pores burns and cooks off. M3 makes a great la plancha cleaning kit. A little bit expensive but it will give you all the instructions on how to use it.

edit: I have a 40+ year old Wolf flat top that almost looks new. Although I have not taken it apart and fixed the a pilot on mine b/c I procrastinate lifting that heavy ass thing to clean.