While making one pot meals like sambar sadam, it is important to add good amount of liquid and seasonings. I have used 1 cup rice and 3/4 cup dal, for this amount i have used almost 5-6 cups of water. It is natural for this kind of one pot meal to turn little cake-y as it rests. If this happens, just before serving, add 1/2 to 1 cup of hot water, adjust salt, add a good tempering of red chilies and curry leaves, give a good stir and serve.